Readme files. The README.md files explain what your project is, and how to install and use it. README.md is the file that is automatically displayed when you open a GitHub repo.
License. Without some sort of licence, the contents of the repository are technically closed. Some allow users of the code to do anything they like with their code - these are known as permissive licences. Examples are the MIT Licence or Apache.
Contributing guide - make a file called CONTRIBUTING.md and guidelines for contributors so they know what they should do if they want to help you out.
Code of Conduct - good projects have codes of conduct to make sure that people are treated well. Github has an Code of Conduct wizard to make it easy to add one.
Issues - use GitHub issues to record and discuss tasks.
(This list was largely inspired by Mozilla’s open leadership 101.