readme.txt updated



git-svn-id: https://plugins.svn.wordpress.org/shortpixel-image-optimiser/trunk@1034378 b8457f37-d9ea-0310-8a92-e5e31aec5664
small edits



git-svn-id: https://plugins.svn.wordpress.org/shortpixel-image-optimiser/trunk@1034382 b8457f37-d9ea-0310-8a92-e5e31aec5664