update custom upload support online link to

git-svn-id: https://plugins.svn.wordpress.org/wechat-subscribers-lite/trunk@997827 b8457f37-d9ea-0310-8a92-e5e31aec5664
update readme

git-svn-id: https://plugins.svn.wordpress.org/wechat-subscribers-lite/trunk@997829 b8457f37-d9ea-0310-8a92-e5e31aec5664