documentation: add links to SELinux resources

Add links to the SELinux kernel subsystem README.md file, the
SELinux kernel wiki, and the SELinux userspace wiki to the
SELinux guide.

Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
[PM: spacing and style corrections, subject tweak]
Signed-off-by: Paul Moore <paul@paul-moore.com>