readme update

git-svn-id: https://plugins.svn.wordpress.org/responsive-widgets/trunk@873119 b8457f37-d9ea-0310-8a92-e5e31aec5664
screenshot fix

git-svn-id: https://plugins.svn.wordpress.org/responsive-widgets/trunk@873120 b8457f37-d9ea-0310-8a92-e5e31aec5664