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>

MAINTAINERS

@@ -7676,6 +7676,7 @@ 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>