readme update

git-svn-id: https://plugins.svn.wordpress.org/nextgen-facebook/trunk@933734 b8457f37-d9ea-0310-8a92-e5e31aec5664
v7.5.1

git-svn-id: https://plugins.svn.wordpress.org/nextgen-facebook/trunk@933736 b8457f37-d9ea-0310-8a92-e5e31aec5664