update os requirements

Author: MagicalTux

os_requirements.txt

@@ -5,4 +5,28 @@ If you want to submit and/or maintain distribution of your choice for official S
 * Set user to autologin (we try to achieve this in firstrun script but if you see difference not applying for your OS, be sure to do it).
 * Disable power saving (screensavers, sleep, automatic logout etc).
 * Disable logout option.
-* Shells images are running on our Shells kernel, if you want your images to be ran by your distro kernel, you will need to supply that kernel, a way to resize the disk, a bootloader, the kernel parameters etc.
+
+## Naming
+
+All images are typically named as `distribution-version-variation`. For
+example Ubuntu Focal with Ubuntu Desktop installed would be called
+`ubuntu-focal-ubuntu-desktop`. All images providing a desktop environment must
+end in `-desktop`.
+
+## Custom kernel
+
+By default Shells boots all images with the Shells kernel, which is a LTS
+vanilla kernel built by the Shells team. This kernel comes with an initrd that
+will perform a number of operations prior to booting the OS, such as ensuring
+the modules are installed for the running kernel, resizing the disk partition
+to use the whole disk, setup a swap partition, etc.
+
+You can however supply your own kernel but will need to ensure the following:
+
+* The image must be a 8GB or less BIOS-bootable bootable partition
+* On boot, you should check if the disk is larger than the partition(s) cover,
+  and resize the disk appropriately. If you have a swap partition you may
+  consider moving it toward the end of the disk.
+* Make sure you have the virtio modules enabled in your kernel, as well as
+  anything required to run qxl if doing an desktop image.
+