added example usage to readme


git-svn-id: https://plugins.svn.wordpress.org/custom-menu-fields/trunk@880682 b8457f37-d9ea-0310-8a92-e5e31aec5664
shortened title of example field access page


git-svn-id: https://plugins.svn.wordpress.org/custom-menu-fields/trunk@880684 b8457f37-d9ea-0310-8a92-e5e31aec5664