readme and add_site_option fix

mitcho, I've added the readme contents as you suggested. In retrospect, I was looking to add a bit too much control in the admin panel.

Also, the add_site_option code was fixed in 3.3.0, so I removed the check that was in place as in current version it throws error for every add - especially on non-multisite installs. I considered doing a version check, but people should be running a recent version anyway.

Thanks!