adding screenshot

git-svn-id: https://plugins.svn.wordpress.org/omnivideo/trunk@852348 b8457f37-d9ea-0310-8a92-e5e31aec5664
editing readme

git-svn-id: https://plugins.svn.wordpress.org/omnivideo/trunk@852349 b8457f37-d9ea-0310-8a92-e5e31aec5664