MAINTAINERS: add stable_kernel_rules.txt to stable maintainer information

This hopefully will help point developers to the proper way that patches
should be submitted for inclusion in the stable kernel releases.

Reported-by: David Howells <dhowells@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>