addons post added

Initial writeup of addon documentation. Please review, edit and publish :)
@bcardarella ok, you have a point in there, I can see this could lead to confusions down the road.

@kristianmandrup do you think we could use a different example instead of relying on the one listed? 

A simple example that comes to mind is a confirmable-button, is already used on the Ember.js guides http://emberjs.com/guides/components/sending-actions-from-components-to-your-application/#toc_sending-parameters-with-an-action