It seems to me that a good README is a summary of all four of his documentation types: it should have a brief explanation, a short how-to on getting started, a tutorial or usage example of the system, and a reference in the form of pointers to further documentation. And something he doesn't mention, graphics and screenshots!
> a good README is a summary of all four of his documentation types
Came here to say/reiterate this. A README is very flexible and can be any of the four types, the decision of which will depend on the contents of the repository and on what the README is trying to accomplish.
