Added Upgrade Notice for 4.0

git-svn-id: https://plugins.svn.wordpress.org/addthis/trunk@1010605 b8457f37-d9ea-0310-8a92-e5e31aec5664
Releasing 4.0

git-svn-id: https://plugins.svn.wordpress.org/addthis/trunk@1010608 b8457f37-d9ea-0310-8a92-e5e31aec5664