readme update

git-svn-id: https://plugins.svn.wordpress.org/nextgen-facebook/trunk@805439 b8457f37-d9ea-0310-8a92-e5e31aec5664
new tag 6.16.0

git-svn-id: https://plugins.svn.wordpress.org/nextgen-facebook/trunk@805442 b8457f37-d9ea-0310-8a92-e5e31aec5664