Add AUTHORS file

[basil-conto]

Closes #50 Cc: @matthew-piziak, @clarete, @shassard, @fabacino, @helgeblod

Let me know if you'd rather not be mentioned or if you have suggestions/criticism for alternative wording/descriptions. I intend to merge this PR in a week if there are no objections in the meantime, but feel free to discuss it even after it's merged.

FYI I have left out several contributors whose contributions amounted to less than 15 LoC, which is what the FSF and GNU project legally consider a "tiny change". I'm not personally against acknowledging more people, though.

[matthew-piziak]

I'm happy to be mentioned. :+1:

[fabacino]

Very nice, thank you!

[basil-conto]

Thanks everyone!