addons post added

Initial writeup of addon documentation. Please review, edit and publish :)
Yeah, I think he did now that I think about it? These docs were in part based on the original @rwblue article http://reefpoints.dockyard.com/2014/06/24/introducing_ember_cli_addons.html

It needs to be updated to reflect the latest API. Why it is better to base the docs on a "real" addon that works with the current version of Ember CLI.