up date to 1.51 read me.

git-svn-id: https://plugins.svn.wordpress.org/wechat-subscribers-lite/trunk@959200 b8457f37-d9ea-0310-8a92-e5e31aec5664
up date to 1.51 read me.

git-svn-id: https://plugins.svn.wordpress.org/wechat-subscribers-lite/trunk@959201 b8457f37-d9ea-0310-8a92-e5e31aec5664