update readme

git-svn-id: https://plugins.svn.wordpress.org/cm-download-manager/trunk@888201 b8457f37-d9ea-0310-8a92-e5e31aec5664
update readme

git-svn-id: https://plugins.svn.wordpress.org/cm-download-manager/trunk@888203 b8457f37-d9ea-0310-8a92-e5e31aec5664