update readme.txt

git-svn-id: https://plugins.svn.wordpress.org/qqworld-auto-save-images/trunk@998388 b8457f37-d9ea-0310-8a92-e5e31aec5664
update to v1.5.1

git-svn-id: https://plugins.svn.wordpress.org/qqworld-auto-save-images/trunk@998390 b8457f37-d9ea-0310-8a92-e5e31aec5664