Updated readme.txt to note additional changes

git-svn-id: https://plugins.svn.wordpress.org/register-plus-redux/trunk@1003351 b8457f37-d9ea-0310-8a92-e5e31aec5664
Prep 4.0.1 release to publish fix around is_wp_error

git-svn-id: https://plugins.svn.wordpress.org/register-plus-redux/trunk@1003353 b8457f37-d9ea-0310-8a92-e5e31aec5664