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. 
```
$ curl -v https://air.mozilla.org/contribute.json > /dev/null
....
< Content-Type: application/json
```
I'm glad it opens in the browser and doesn't force me to download it. Also I have jsonview installed in Firefox so it's pretty to look at without having to download it and open in an editor.