update readme.txt

git-svn-id: https://plugins.svn.wordpress.org/captcha-bank/trunk@1023521 b8457f37-d9ea-0310-8a92-e5e31aec5664
update readme.txt

git-svn-id: https://plugins.svn.wordpress.org/captcha-bank/trunk@1023525 b8457f37-d9ea-0310-8a92-e5e31aec5664