Add link to prior work on promise-as-monad

Just watched the talk, really enjoyed it, thank you very much. I was
waiting for you to get to the promise-as-monad punchline the whole time,
and you delivered. But when you sort of took credit for the idea, I had
to regoogle some of the things I had been reading about the
concept--sometime over a year ago. Check out the fantastic work at the
URL I added. To your credit, though, these are certainly thoughts that
deserver a much wider audience. Thanks again for speaking on them.