Add warning and recommend native promises

[benjamingr]

Hey @kriskowal ,

Is it OK if I add a warning to the readme recommending native promises?

A discussion with @domenic yesterday reminded me a lot of people still use promise libraries. I've made the Bluebird warning much sterner.

I don't have the same sense of collaboratorship here as I do in Bluebird since I was made a collaborator pretty much after the development of the library has stopped - so I don't feel like I have the mandate to add the warning without asking first.

[kriskowal]

I’m open to having a strong recommendation to use Promise added to our README.