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 
What is the rule you're wanting here? I haven't enforced anything because I haven't found anything I loved when it comes to multiple line calls (components that pass in lots of things).