readme and assets update

git-svn-id: https://plugins.svn.wordpress.org/wpsso/trunk@991087 b8457f37-d9ea-0310-8a92-e5e31aec5664
v2.6.4

git-svn-id: https://plugins.svn.wordpress.org/wpsso/trunk@991093 b8457f37-d9ea-0310-8a92-e5e31aec5664