This is the first version of an introductory document that should help
developers who want to get started with this package.

I recently started poking around this part of the compiler, and was
confused by a few basic ideas such as memory arguments. I also hadn't
heard about GOSSAFUNC until another developer pointed it out. Both of
those are essential if one wants to do any non-trivial work here.

This document can of course be expanded with more pointers and tips to
better understand this package's code and behavior. Its intent is not to
cover all of its features; but it should be enough for most developers
to start playing with it without extensive compiler experience.

Change-Id: Ifd2d047fbd038ab50f4625a15c4d49932b42fd66
Reviewed-on: https://go-review.googlesource.com/99715Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>