updated readme.txt to show tested up to WP 3.9

git-svn-id: https://plugins.svn.wordpress.org/nonprofit-board-management/trunk@923217 b8457f37-d9ea-0310-8a92-e5e31aec5664
Plugin tested to work with WorPress up to version 3.9

git-svn-id: https://plugins.svn.wordpress.org/nonprofit-board-management/trunk@923219 b8457f37-d9ea-0310-8a92-e5e31aec5664