Update readme

git-svn-id: https://plugins.svn.wordpress.org/really-simple-captcha/trunk@756966 b8457f37-d9ea-0310-8a92-e5e31aec5664
ready for 1.7 release

git-svn-id: https://plugins.svn.wordpress.org/really-simple-captcha/trunk@756967 b8457f37-d9ea-0310-8a92-e5e31aec5664