Restructure README
Restructure README

The major changes here are:

* Narrow the focus to programmers, with lots of links for other folks.
  Rationale: the reader is already looking at the code repository, and the
  README is traditionally used by programmers for a very specific purpose.
* Introduce stack.

akegalj/snowdrift,akegalj/snowdrift,snowdriftcoop/snowdrift