Add appendix for list of undocumented features

[mrhota]

Re: #9

r? @steveklabnik

CC @chriskrycho

[steveklabnik]

This is an interesting idea; @chriskrycho do you think this is helpful?

[mrhota]

@steveklabnik I think this is actually a requirement of the RFC for #9. The quotation at the top of the issue mentions it, with another mention in section C.1.

[chriskrycho]

Quite! So much so that RFC 1636 said we'd do exactly this:

Updating the reference should proceed stepwise:

1. Begin by adding an appendix in the reference with links to all accepted RFCs which have been implemented but are not yet referenced in the documentation.
2. As the reference material is written for each of those RFC features, remove it from that appendix.

[steveklabnik]

Oh hey, I forgot! 😓 Let's do it!

[chriskrycho]

I've integrated these into the tracking issue; thanks again for the help!