Added Screenshots and Updated readme.txt

git-svn-id: https://plugins.svn.wordpress.org/mailpoet-gravity-forms-add-on/branches/2.0.0@878901 b8457f37-d9ea-0310-8a92-e5e31aec5664
Updated readme.txt

git-svn-id: https://plugins.svn.wordpress.org/mailpoet-gravity-forms-add-on/branches/2.0.0@878902 b8457f37-d9ea-0310-8a92-e5e31aec5664