README files are the welcome mat for your project. They are the first thing new visitors to your project will see and thus are part of a set of really important documents to make potential contributors feel welcome and invite them to get involved. Your README file should cover:
- What you are doing, for who, and why.
- What makes your project special and exciting.
- How to get started.
- Where to find key resources.
An Open Canvas can be a good tool to help you clarify what you want to achieve with your project and what you should cover in your README.