Doc/stable rules: add new cherry-pick logic

- it is possible to submit patches for the stable queue without sending
  them directly stable@kernel.org. If the tag (Cc: stable@kernel.org) is
  available in the sign-off area than hpa's script will filter them into
  the stable mailbox once it hits Linus' tree.
- Patches which require others to be applied first can be also specified.
  This was discussued in http://lkml.org/lkml/2009/11/9/474



Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Cc: Brandon Philips <brandon@ifup.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>