allow nesting even if not xml post

git-svn-id: https://plugins.svn.wordpress.org/forms-3rd-party-xpost/trunk@986925 b8457f37-d9ea-0310-8a92-e5e31aec5664
readme text update, trying to better indicate that you can nest values

git-svn-id: https://plugins.svn.wordpress.org/forms-3rd-party-xpost/trunk@986926 b8457f37-d9ea-0310-8a92-e5e31aec5664