improve readme.txt

git-svn-id: https://plugins.svn.wordpress.org/ridiculously-responsive-social-sharing-buttons/trunk@927430 b8457f37-d9ea-0310-8a92-e5e31aec5664
add tags to readme.txt

git-svn-id: https://plugins.svn.wordpress.org/ridiculously-responsive-social-sharing-buttons/trunk@927431 b8457f37-d9ea-0310-8a92-e5e31aec5664