Add first pass at Ember guidelines

These are the first few things I've seen that are consistently useful. All feedback and input is appreciated.

cc: @jsteiner @BlakeWilliams @seanpdoyle @halogenandtoast @creuter @MattMSumner @JoelQ 
I'm not sure how to phrase this. We shouldn't be making an actual API call anywhere outside of a model in my opinion but what I was trying to mean here was do not make calls like `save` or `destroyRecord` outside of the route. Any idea how to phrase that?