addons post added

Initial writeup of addon documentation. Please review, edit and publish :)
Also a mention of the dependency ordering would be nice. Mainly that you can provide a `before` or `after` key(array or string) of other addon's package names to be ran before or after.

[relevant code here](https://github.com/stefanpenner/ember-cli/blob/master/lib/models/project.js#L136-L157)