Release notes: BD xlator known issues

Change-Id: Ic6ced2ce4a7e9c0ff6333edcac4dfbdfa161b047
Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
Reviewed-on: http://review.gluster.org/5334
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>