update readme

git-svn-id: https://plugins.svn.wordpress.org/cm-ad-changer/trunk@903775 b8457f37-d9ea-0310-8a92-e5e31aec5664
update readme.txt

git-svn-id: https://plugins.svn.wordpress.org/cm-ad-changer/trunk@903776 b8457f37-d9ea-0310-8a92-e5e31aec5664