Promises guide updates + promises in web animations

Tonight I added a section to the promises guide on using promises for state transitions, which is a pattern we've seen emerge in a few specs and feature requests, and I thought would be good to have an authoritative source for:

https://github.com/w3ctag/promises-guide#more-general-state-transitions

In fact, now that that is available, I can link to it in my proposed addition to Sergey's web animations review:

https://github.com/w3ctag/spec-reviews/pull/29

I think this is a good pattern for us to coordinate and encourage across specs, and wanted to let people know. I'd also appreciate any feedback people have on the guide or the review text.