CONTRIBUTING.md tells a newcomer how to set up, test, and submit a change
A contributor who cannot run the tests sends a patch you have to fix yourself. The three commands that get them from clone to green cost you one paragraph and save every future contributor an hour.
Observable check
CONTRIBUTING.md exists at the repository root, in .github/, or in docs/ on GitHub, or at the repository root on GitLab, and states the setup command, the test command, and how to open a pull request or merge request.