For those who are interested in some of the details of the work that's going on, Lin Clark's recent talk on "A Cartoon Intro to Fiber" at ReactConf 2017 is excellent [0]. There's a number of other existing writeups and resources on how Fiber works [1] as well. The roadmap for 15.5 and 16.0 migration is at [2], and the follow-up issue discussing the plan for the "addons" packages is at [3].
I'll also toss out my usual reminder that I keep a big list of links to high-quality tutorials and articles on React, Redux, and related topics, at https://github.com/markerikson/react-redux-links . Specifically intended to be a great starting point for anyone trying to learn the ecosystem, as well as a solid source of good info on more advanced topics. Finally, the Reactiflux chat channels on Discord are a great place to hang out, ask questions, and learn. The invite link is at https://www.reactiflux.com .
[0] https://www.youtube.com/watch?v=ZCuYPiUIONs
[1] https://github.com/markerikson/react-redux-links/blob/master...