Removed screenshots from core

git-svn-id: https://plugins.svn.wordpress.org/slideshow-gallery/trunk@988164 b8457f37-d9ea-0310-8a92-e5e31aec5664
removed readme.md file

git-svn-id: https://plugins.svn.wordpress.org/slideshow-gallery/trunk@988165 b8457f37-d9ea-0310-8a92-e5e31aec5664