update readme.txt

git-svn-id: https://plugins.svn.wordpress.org/vp-scroll-to-top/trunk@913372 b8457f37-d9ea-0310-8a92-e5e31aec5664
update readme.txt

git-svn-id: https://plugins.svn.wordpress.org/vp-scroll-to-top/trunk@913375 b8457f37-d9ea-0310-8a92-e5e31aec5664