README.md is a minimum qualifying requirement to call a project well documented. It is basically an introduction to your project.
README file typically written in Markdown and contains:
A short description of the problem the project solves.
A list of features that make your project great at solving the given problem.
This section is basically a usage guide. Include installation and usage instructions here.
This is also a good place to add some code examples as well.
Discuss coding style and guidelines. Include steps to run test suite and introduce the contributor to any CI validation (if already setup)
List ways in which your end users can seek help. Email, Slack and JIRA are some good ways of allowing your users and developers to file bugs and ask for help.
💡 Focus on building a community of expert users and developers, so that you don't have to support your project all the time.
Read about licenses here.
I found a collection of awesome-readmes which you might enjoy as well!