add screenshot

git-svn-id: https://plugins.svn.wordpress.org/widget4call/trunk@1000494 b8457f37-d9ea-0310-8a92-e5e31aec5664
change readme

git-svn-id: https://plugins.svn.wordpress.org/widget4call/trunk@1000496 b8457f37-d9ea-0310-8a92-e5e31aec5664