doc: release-notes for 3.4.0

Change-Id: I8245e35198b9c5990fa70dea2b2b03e1c227e411
BUG: 825906
Reviewed-on: http://review.gluster.org/5329
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>