add a contribute.json file (bug 1074452)

I did my best to find all the data to populate the json file, but l'm assuming I'll need to make a change or two. 
my suggestion was wrong. JSON should never be `text/plain`.

and `application/json` is fine. it shouldn't automatically download in your browser (but it will if it's something like `application/x-web-app-manifest+json`).

anyway, headers look fine and it loads in the browser fine:
![screenshot 2014-12-09 10 36 14](https://cloud.githubusercontent.com/assets/203725/5363249/5cffd344-7f8f-11e4-8c81-93b4563a0b5c.png)
![screenshot 2014-12-09 10 29 49](https://cloud.githubusercontent.com/assets/203725/5363248/5cfebc16-7f8f-11e4-8763-56b3628413f5.png)