Update readme.txt for v. 1.6.0

git-svn-id: https://plugins.svn.wordpress.org/square-sidebars/trunk@964039 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update readme.txt for v. 1.6.0

git-svn-id: https://plugins.svn.wordpress.org/square-sidebars/trunk@964040 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update readme.txt for v. 1.6.0

git-svn-id: https://plugins.svn.wordpress.org/square-sidebars/trunk@964045 b8457f37-d9ea-0310-8a92-e5e31aec5664