readme.txt updates

git-svn-id: https://plugins.svn.wordpress.org/wp-maintenance-mode/trunk@976675 b8457f37-d9ea-0310-8a92-e5e31aec5664
readme updates

git-svn-id: https://plugins.svn.wordpress.org/wp-maintenance-mode/trunk@976678 b8457f37-d9ea-0310-8a92-e5e31aec5664