add 2.0 version readme.txt

git-svn-id: https://plugins.svn.wordpress.org/geetest/trunk@1016476 b8457f37-d9ea-0310-8a92-e5e31aec5664
5.0.1.6

git-svn-id: https://plugins.svn.wordpress.org/geetest/trunk@1016479 b8457f37-d9ea-0310-8a92-e5e31aec5664