readme update

git-svn-id: https://plugins.svn.wordpress.org/custom-share-buttons/trunk@917092 b8457f37-d9ea-0310-8a92-e5e31aec5664
readme update

git-svn-id: https://plugins.svn.wordpress.org/custom-share-buttons/trunk@917093 b8457f37-d9ea-0310-8a92-e5e31aec5664