Add appendix section for "newest features" (RFC 1636)

[chriskrycho]

During the discussion of RFC #1636, @carols10cents suggested one solution to dealing with the "features accepted and/or implemented after the paper book goes to print":

Idea: what if, post-print or without intent to incorporate into the printed version, we add to the book a chapter or appendix for sections on "newest features"? I'd be totally fine with anyone contributing docs of any style to be smaller vignettes on various features in this chapter or appendix. This way, we at least have some documentation available in with the book where people would look for it, but we wouldn't be gating on @steveklabnik or I or whoever else to ensure narrative/voice. As features stabilize, become more widely used, and integrate more tightly with the language as a whole, we could work this content into the main book.

This seemed an eminently sane suggestion; I'm just filing the issue here so the idea doesn't get lost in the midst of many other tasks and that long thread!

[steveklabnik]

Hm, interesting. Kinda reminds me of the Unstable book.