Add contribute.json

@jgmize r?
Yeah. I did it this way for 2 reasons:

1. peterbe did it this way for Air.m.o and it's working fine, and the code was there.
2. I wasn't immediately sure how to reference the file in the apache conf as a relative URL. AFAIK no other files outside of the `media` dir are referenced, and I didn't want to have to have IT add a thing for every environment's specific path to their apache conf. I could have researched that more, but decided to go this way for these and the reasons you mentioned.