updated readme. less text.

git-svn-id: https://plugins.svn.wordpress.org/mailchimp-for-wp/trunk@814813 b8457f37-d9ea-0310-8a92-e5e31aec5664
changed heading in readme.txt

git-svn-id: https://plugins.svn.wordpress.org/mailchimp-for-wp/trunk@814814 b8457f37-d9ea-0310-8a92-e5e31aec5664