update readme

git-svn-id: https://plugins.svn.wordpress.org/simple-course-creator/trunk@894444 b8457f37-d9ea-0310-8a92-e5e31aec5664
remove unnecessary screenshots

git-svn-id: https://plugins.svn.wordpress.org/simple-course-creator/trunk@894446 b8457f37-d9ea-0310-8a92-e5e31aec5664