Added missing release note about timestamps changing to gmt.

Change-Id: Ic262ea513525faa859f5fba11f4bd040d4694269
Signed-off-by: Joe Julian <me@joejulian.name>
Reviewed-on: http://review.gluster.org/5338
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>