When releasing a project or a library into the open, what documentation is the best to include?

Are there any guidelines to writing good, but not too specific, documentation and examples on how to use open source code. Often, I find good libraries, but the only documentation is in the code, making it much harder to work with.

A general overview, example usage, a tutorial, and basic project layout / goals seem to be a few popular options. However, if it's a single-developer project and just starting out, the luxury of writing all that isn't there. Which is the best advice for starting documentation of a project?