Updating releasenotes.html

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.0.6-alpha@1512842 13f79535-47bb-0310-9956-ffa450edef68
Setting "optimistic release date" in the CHANGES.txt

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.0.6-alpha@1512843 13f79535-47bb-0310-9956-ffa450edef68