readme update

git-svn-id: https://plugins.svn.wordpress.org/nextgen-facebook/trunk@807551 b8457f37-d9ea-0310-8a92-e5e31aec5664
added check for -1 meta tag values to ignore those meta tags

git-svn-id: https://plugins.svn.wordpress.org/nextgen-facebook/trunk@807552 b8457f37-d9ea-0310-8a92-e5e31aec5664