Add contributors section, format readme

I added a contributors section as a very small way of saying thanks for contributing to hologram.

Please let me know if you'd like to be removed or if you want your name/link to be different. (The relevant diff is at the bottom, sorry for all the formatting changes)

Thanks again for help!

@jho406
@elanakoren
@kmayer
@ostinelli
@stubbornella
@BrentWheeldon