Update readme

git-svn-id: https://plugins.svn.wordpress.org/contact-form-7/trunk@758197 b8457f37-d9ea-0310-8a92-e5e31aec5664
Ready for release 3.5.2

git-svn-id: https://plugins.svn.wordpress.org/contact-form-7/trunk@758198 b8457f37-d9ea-0310-8a92-e5e31aec5664