Adding in Explicit Inverse

This is a literal copy-paste from http://emberjs.com/blog/2012/11/02/this-week-in-ember-js.html. I've encountered at least 1 person asking this in the IRC and I can see that I would need this information too. Instead of going over blog posts, this seems worthy of being in the guides for models.
Merge pull request #484 from rystraum/patch-1

Adding in Explicit Inverse