readme update

git-svn-id: https://plugins.svn.wordpress.org/nextgen-facebook/trunk@832962 b8457f37-d9ea-0310-8a92-e5e31aec5664
6.21rc1

git-svn-id: https://plugins.svn.wordpress.org/nextgen-facebook/trunk@832963 b8457f37-d9ea-0310-8a92-e5e31aec5664
6.21rc1

git-svn-id: https://plugins.svn.wordpress.org/nextgen-facebook/trunk@832965 b8457f37-d9ea-0310-8a92-e5e31aec5664