Adding 2.0.6-alpha release to the CHANGES.txt

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.3@1558222 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5677. Merge 1558217 from trunk

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.3@1558223 13f79535-47bb-0310-9956-ffa450edef68