Always include alt tag on images for HTML5 validation

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@894154 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update readme

git-svn-id: https://plugins.svn.wordpress.org/ml-slider/trunk@894155 b8457f37-d9ea-0310-8a92-e5e31aec5664