Add documentation about how to create profiles

README.profile.rst:
Add information on all necessary components of a profile:

* profiledef.sh
* packages.arch
* airootfs/
* efiboot/
* isolinux/
* syslinux/

Fix wording for pacman.conf documentation

README.rst:
Add a link to README.profile.rst.

Fixes #38

2 files changed, 122 insertions, 3 deletions

diff --git a/README.profile.rst b/README.profile.rst
index 7e8e177..94cecdd 100644
--- a/README.profile.rst
+++ b/README.profile.rst
@@ -4,6 +4,60 @@ profile
 
 An archiso profile consists of several configuration files and a directory for files to be added to the resulting image.
 
+  .. code:: bash
+
+    profile
+    |- airootfs/
+    |- efiboot/
+    |- isolinux/
+    |- syslinux/
+    |- packages.arch
+    |- pacman.conf
+    \- profiledef.sh
+
+The required files and directories are explained in the following sections.
+
+profiledef.sh
+=============
+
+This file describes several attributes of the resulting image and is a place for customization to the general behavior
+of the image.
+
+The image file is constructed from some of the variables in **profiledef.sh**: `<iso_name>-<iso_version>-<arch>.iso`
+(e.g. `archlinux-202010-x86_64.iso`).
+
+* `iso_name`: The first part of the name of the resulting image (defaults to `mkarchiso`)
+* `iso_label`: The ISO's volume label (defaults to `MKARCHISO`)
+* `iso_publisher`: A free-form string that states the publisher of the resulting image (defaults to `mkarchiso`)
+* `iso_application`: A free-form string that states the application (i.e. its use-case) of the resulting image (defaults
+  to `mkarchiso iso`)
+* `iso_version`: A string that states the version of the resulting image (defaults to `""`)
+* `install_dir`: A string (maximum eight characters long, which **must** consist of `[a-z0-9]`) that states the
+  directory on the resulting image into which all files will be installed (defaults to `mkarchiso`)
+* `bootmodes`: A list of strings, that state the supported boot modes of the resulting image. Only the following are
+  understood:
+  - `bios.syslinux.mbr`: Syslinux for x86 BIOS booting from a disk
+  - `bios.syslinux.eltorito`: Syslinux for x86 BIOS booting from an optical disc
+  - `uefi-x64.systemd-boot.esp`: Systemd-boot for x86_64 UEFI booting from a disk
+  - `uefi-x64.systemd-boot.eltorito`: Systemd-boot for x86_64 UEFI booting from an optical disc
+* `arch`: The architecture (e.g. `x86_64`) to build the image for. This is also used to resolve the name of the packages
+  file (e.g. `packages.x86_64`)
+* `pacman_conf`: The `pacman.conf` to use to install packages to the work directory when creating the image (defaults to
+  the host's `/etc/pacman.conf`)
+
+packages.arch
+=============
+
+All packages to be installed into the environment of the image have to be listed in an architecture specific file (e.g.
+`packages.x86_64`), which resides top-level in the profile.
+
+Packages have to be listed one per line. Lines starting with a `#` and blank lines are ignored.
+
+  .. note::
+
+    The **mkinitcpio** and **mkinitcpio-archiso** packages are mandatory (see `#30
+    <https://gitlab.archlinux.org/archlinux/archiso/-/issues/30>`_).
+
 pacman.conf
 ===========
 
@@ -13,12 +67,75 @@ Some configuration options will not be used or will be modified:
 
 * `CacheDir`: the profile's option is **only** used if it is not the default (i.e. `/var/cache/pacman/pkg`) and if it is
   not the same as the system's option. In all other cases the system's pacman cache is used.
-* `HookDir`: it is **always** set to the `/etc/pacman.d/hooks` airootfs directory in the work directories airootfs to
-  allow modification via the profile and ensure interoparability with hosts using dracut (see #73 for further
-  information)
+* `HookDir`: it is **always** set to the `/etc/pacman.d/hooks` directory in the work directory's airootfs to allow
+  modification via the profile and ensure interoparability with hosts using dracut (see `#73
+  <https://gitlab.archlinux.org/archlinux/archiso/-/issues/73>`_)
 * `RootDir`: it is **always** removed, as setting it explicitely otherwise refers to the host's root filesystem (see
   `man 8 pacman` for further information on the `-r` option used by `pacstrap`)
 * `LogFile`: it is **always** removed, as setting it explicitely otherwise refers to the host's pacman log file (see
   `man 8 pacman` for further information on the `-r` option used by `pacstrap`)
 * `DBPath`: it is **always** removed, as setting it explicitely otherwise refers to the host's pacman database (see
   `man 8 pacman` for further information on the `-r` option used by `pacstrap`)
+
+airootfs
+========
+
+This - optional - directory may contain files and directories that will be copied to the work directory of the resulting
+image's root filesystem.
+The files are copied before packages are being installed to work directory location.
+Ownership of files and directories from the profile's `airootfs` directory are not preserved (permissions are currently
+the same as in the profile's `airootfs` - see `#61 <https://gitlab.archlinux.org/archlinux/archiso/-/issues/73>`_).
+
+With this overlay structure it is possible to e.g. create users and set passwords for them, by providing
+`airootfs/etc/passwd`, `airootfs/etc/shadow`, `airootfs/etc/gshadow` (see `man 5 passwd`, `man 5 shadow` and `man 5
+gshadow` respectively).
+If user home directories exist in the profile's `airootfs`, their ownership and (and top-level) permissions will be
+altered according to the provided information in the password file.
+
+Boot loader configuration
+=========================
+
+A profile may contain configuration for several boot loaders. These reside in specific top-level directories, which are
+explained in the following subsections.
+
+The following *custom template identifiers* are understood and will be replaced according to the assignments of the
+respective variables in **profiledef.sh**:
+
+* `%ARCHISO_LABEL%`: Set this using the `iso_label` variable in **profiledef.sh**
+* `%INSTALL_DIR%`: Set this using the `iso_label` variable in **profiledef.sh**
+* `%ARCH%`: Set this using the `arch` variable in **profiledef.sh**
+
+
+efiboot
+-------
+
+This directory is mandatory when the `uefi-x64.systemd-boot.esp` or `uefi-x64.systemd-boot.eltorito` bootmodes are
+selected in **profiledef.sh**. It contains configuration for `systemd-boot
+<https://www.freedesktop.org/wiki/Software/systemd/systemd-boot/>`_.
+
+  .. note::
+
+    The directory is a top-level representation of the systemd-boot configuration directories and files found in the
+    root of an EFI system partition.
+
+The *custom template identifiers* are **only** understood in the boot loader entry `.conf` files (i.e. **not** in
+`loader.conf`).
+
+isolinux
+--------
+
+This directory is mandatory when the `bios.syslinux.eltorito` bootmode is selected in **profiledef.sh**.
+It contains configuration for `isolinux <https://wiki.syslinux.org/wiki/index.php?title=ISOLINUX>`_ used in the resuling
+image.
+
+The *custom template identifiers* are understood in all `.cfg` files in this directory.
+
+syslinux
+--------
+
+This directory is mandatory when the `bios.syslinux.mbr` or the `bios.syslinux.eltorito` bootmodes are selected in
+**profiledef.sh**.
+It contains configuration files for `syslinux <https://wiki.syslinux.org/wiki/index.php?title=SYSLINUX>`_ or `pxelinux
+<https://wiki.syslinux.org/wiki/index.php?title=PXELINUX>`_ used in the resuling image.
+
+The *custom template identifiers* are understood in all `.cfg` files in this directory.
diff --git a/README.rst b/README.rst
index 1553d77..03420b4 100644
--- a/README.rst
+++ b/README.rst
@@ -35,6 +35,8 @@ custom live media, **releng** is used to create the monthly installation medium.
 They can be found below `configs/baseline/ <configs/baseline/>`_  and `configs/releng/ <configs/releng/>`_
 (respectively). Both profiles are defined by files to be placed into overlays (e.g. *airootfs* -> *the image's /*).
 
+Read `README.profile.rst <README.profile.rst>`_ to learn more about how to create profiles.
+
 Create images
 =============