update readme.txt

git-svn-id: https://plugins.svn.wordpress.org/jetpack-sharing/trunk@823023 b8457f37-d9ea-0310-8a92-e5e31aec5664
update to 2.7

git-svn-id: https://plugins.svn.wordpress.org/jetpack-sharing/trunk@823995 b8457f37-d9ea-0310-8a92-e5e31aec5664