added premium notice to readme files

git-svn-id: https://plugins.svn.wordpress.org/related-posts-for-wp/trunk@1007512 b8457f37-d9ea-0310-8a92-e5e31aec5664
added premium notice to readme files

git-svn-id: https://plugins.svn.wordpress.org/related-posts-for-wp/trunk@1007515 b8457f37-d9ea-0310-8a92-e5e31aec5664