As others have noted, README should be simple and short, but a good README can save time especially if it's for something like command-line parameter parsing library.
Here's what I think it should include:
name of the projects and all sub-modules and libraries (sometimes they are named different and very confusing to new users)
descriptions of all the project, and all sub-modules and libraries
5-line code snippet on how its used (if it's a library)
copyright and licensing information (or "Read LICENSE")
instruction to grab the documentation
instructions to install, configure, and to run the programs