doc: Adding documentation for file-snapshot

Change-Id: I52b70a2fb24f5e3f802f0ce8b1c50102c267e218
BUG: 1086750
Signed-off-by: Kasturi Narra <knarra@redhat.com>
Reviewed-on: http://review.gluster.org/7487
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>