HBASE-7410 add snapshot/clone/restore/export docs to ref guide

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1468977 13f79535-47bb-0310-9956-ffa450edef68
New CHANGES.txt

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1468999 13f79535-47bb-0310-9956-ffa450edef68