I did this for myself but thought somebody else might find it useful. I know there is another similar workflow flying around that gives you more info on the song being rated and you can use half stars. If that's what you need, by all means use that one.
I just wanted a quick way to rate songs with minimal interaction (there is only one banner in Notification Center confirming your rating). There is no need to run additional scripts, jump to the command line or install any additional package. This is as simple as it gets.
Usage: Invoke Alfred, hit "rtg" then Enter followed by the star rating.
You can download the workflow from here. http://goo.gl/g8p5w
