runtime: expand HACKING.md

This adds high-level descriptions of the scheduler structures, the
user and system stacks, error handling, and synchronization.

Change-Id: I1eed97c6dd4a6e3d351279e967b11c6e64898356
Reviewed-on: https://go-review.googlesource.com/34290


Reviewed-by: Rick Hudson <rlh@golang.org>