mkarchiso: add version information

To date the iso version was used for iso volume information and iso file name.
In my custom builds I do use it a lot more:

* Inside the root fs: The system knows about its own version. I use this to:
  -> report the version to a server (poor man's inventory)
  -> let the system update itself

* On the iso fs: The files are served via rsync, running systems transfer
  version file first to check for available update.

* A grub environment file on the iso fs: Booting the iso from grub allows
  to create cow directory per version:

    loopback loop archlinux.iso
    load_env -f (loop)/arch/grubenv
    linux (loop)/arch/boot/x86_64/vmlinuz-linux ... \
        cow_directory=archlinux/${VERSION} ...

So let's just create these files.

1 files changed, 13 insertions, 0 deletions

diff --git a/README.rst b/README.rst
index 03420b4..00e79e1 100644
--- a/README.rst
+++ b/README.rst
@@ -118,6 +118,19 @@ Optionally install archiso's mkinitcpio hooks:
 
     make install-initcpio
 
+Optional Features
+=================
+
+The iso image contains a grub environment block holding the iso version. This allows to boot the iso image from grub
+with a version specific cow directory to mitigate overlay clashes.
+
+  .. code:: grub
+      loopback loop archlinux.iso
+      load_env -f (loop)/arch/grubenv
+      linux (loop)/arch/boot/x86_64/vmlinuz-linux ... \
+          cow_directory=archlinux/${VERSION} ...
+      initrd (loop)/arch/boot/x86_64/initramfs-linux-lts.img
+
 Contribute
 ==========