Updated readme.txt (added screenshots)

git-svn-id: https://plugins.svn.wordpress.org/ultimate-social-media-icons/trunk@994730 b8457f37-d9ea-0310-8a92-e5e31aec5664
Updated readme.txt, so that screenshots get displayed (hopefully)

git-svn-id: https://plugins.svn.wordpress.org/ultimate-social-media-icons/trunk@994734 b8457f37-d9ea-0310-8a92-e5e31aec5664