docs: document our stable process

Some pointers on how to get a patch into stable.

[contains some suggestions by mdroth and eblake]
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>