MAINTAINERS: add stable_kernel_rules.txt to stable maintainer information

commit 7b175c46

 upstream.

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>

MAINTAINERS

@@ -6390,6 +6390,7 @@ STABLE BRANCH
 M:	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
 L:	stable@vger.kernel.org
 S:	Supported
+F:	Documentation/stable_kernel_rules.txt
 
 STAGING SUBSYSTEM
 M:	Greg Kroah-Hartman <gregkh@linuxfoundation.org>