Add release notes for 3.5.1

Change-Id: I7e46f470779a6cb335fcb24b836b32fcd5e14c10
BUG: 1071800
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/8036
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>