update readme

git-svn-id: https://plugins.svn.wordpress.org/announcer/trunk@909101 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update readme

git-svn-id: https://plugins.svn.wordpress.org/announcer/trunk@909104 b8457f37-d9ea-0310-8a92-e5e31aec5664