doc: Adding user documentation for WORM feature

Change-Id: Ic31c8305254dd00eae986fbb322cac3761b5d330
BUG: 1086760
Signed-off-by: Kasturi Narra <knarra@redhat.com>
Reviewed-on: http://review.gluster.org/7530
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Lalatendu Mohanty <lmohanty@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>