PROTOCOL.md: Formatting and Updates.

* Break long sections into multiple lines since it makes them more readable
  when not rendered.
* Add links to the relevant RFCs.
* Add mailto: links for authors.
* Prefer using `` (pre-format) for literals, reference to literals, and
  reference to things in the figure.
Merge pull request #1 from udoprog/wip-protocol

PROTOCOL.md: Formatting and Updates.