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. 
to keep things simple, why don't we just serve the file like we do for the [`robots.txt` view](https://github.com/rachelhathaway/zamboni/blob/1074452/mkt/site/views.py#L117-L120)?

(or it could even be a `json_view`, but that seems like overkill.)