add upgrade notice

git-svn-id: https://plugins.svn.wordpress.org/mediamaster-reader-rss/trunk@861558 b8457f37-d9ea-0310-8a92-e5e31aec5664
upgrade notice link

git-svn-id: https://plugins.svn.wordpress.org/mediamaster-reader-rss/trunk@861562 b8457f37-d9ea-0310-8a92-e5e31aec5664