Add contribute.json file.

[Osmose]

Add a contribute.json file a la http://contribute.paas.allizom.org/.

I'm not entirely sure this project needs or wants a contribute.json file, but I figured it was worth submitting one to start the discussion.

[Pomax]

As the example app, there's unlikely going to be a great deal of external contribution going on, but in terms of "clone this repo to start a new project" it's an interesting idea to have that come with a contribute.json in the app's hosting dir so that it's simply there when the app runs online. Instead of a contribute.json specifically for "this project", I suspect it would be far more valuable to have a contribute-.json builder based off of the package.json and a descriptor file that people can fill out when they clone/yeoman this project and then when they run gulp build or the like, the contribute.json simply gets generated into app/public (or whatever that directory ends up being called if we rename it in the future).