lsm_selinux: Convert to Markdown

Add a TOC to aid navigation and convert to markdown.

To convert to markdown required updating and moving text. Also
added comments regarding later LSM support (such as the
include/linux/lsm_hook_defs.h file etc.).

Signed-off-by: Richard Haines <[email protected]>
Signed-off-by: Paul Moore <[email protected]>

Author: Richard Haines