doc: Point to release branch builds

Change-Id: I2b4e759780ac3c31ffef703b0bc6fc4b1112f25c
Signed-off-by: Luis Pabon <lpabon@redhat.com>
Reviewed-on: http://review.gluster.org/5330
Reviewed-by: Peter Portante <pportant@redhat.com>
Tested-by: Peter Portante <pportant@redhat.com>