Starter kit (created by a Google Code-in student) has served as main introduction for GSoC and Outreachy candidates for more than a year now. I'm also using it in conjunction with this How to contribute page and Annoying little bugs whenever someone sends me an email asking for places to start contributing. I think linking to it here would be useful. What about this:
Grab your starter kit and learn how you can contribute to MediaWiki and the free culture community.