Add stable branch to maintainers file

While helping someone to submit a patch to the stable branch, I noticed
that the stable branch is not listed in the MAINTAINERS file.  This was
after I went there to look for the email addresses for the stable branch
list (stable@kernel.org).

This patch adds the stable branch to the maintainers file so that people
can find where to send patches when they have a fix for the stable team.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>