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. 
The content-type comes from django running this standard python thing:
```python
mimetypes.guess_type(fullpath)
```
Try doing this:
```
$ ./manage.py shell
>>> import mimetypes
>>> mimetypes.guess_type('contribute.json')
```
I'm using django 1.5.9 and python 2.6.8 and I get this:
```
>>> import mimetypes
>>> mimetypes.guess_type('contribute.json')
('application/json', None)
```
I doubt it's going to be a problem once you put this on the servers, which are linux, so I don't think it should be a problem. The content is vastly more important than the content-type.