README
The front-page document of a repository, explaining what the project is and how to use it.
What it tells you about a developer
Clear, thorough READMEs show a developer writes for other humans without being asked. Documentation quality correlates with how well someone onboards teammates and communicates with non-engineers.
GitHub shows a repository's README automatically on the project page, so it is the first thing any visitor reads. Quality varies enormously, from an empty file to a full guide with screenshots and a live demo.