Mark as 2.6.6

git-svn-id: https://plugins.svn.wordpress.org/query-monitor/trunk@865720 b8457f37-d9ea-0310-8a92-e5e31aec5664
Readme update

git-svn-id: https://plugins.svn.wordpress.org/query-monitor/trunk@866403 b8457f37-d9ea-0310-8a92-e5e31aec5664