The general flow of a README:

A section describing what the package does and why it's important (i.e. "what's in it for me" for the user)
A section on how to install and use the package
A section on common configuration options and methods
A section on how to contribute or pointer to CONTRIBUTING.md, notes on the developer's build environment and potential portability problems.
a brief explanation of any unusual top-level directories or files, or other hints for readers to find their way around the source;

Since most all of my projects are Github, you have have some neat markdown block extensions available:

[!CAUTION] [!IMPORTANT] [!NOTE] [!TIP] [!WARNING]

Use these to call out any semantic sections.

readme-writer