A repository should explain itself
A professional repository combines readable source code with documentation, contribution guidance, issue organization, automated checks and an understandable release process.
Recommended baseline
For many projects, establish a README, license where appropriate, contribution guide, security policy, pull request template, issue templates, CI workflow and dependency management configuration.
Optimize for a new contributor
Imagine someone has never seen the project. They should be able to discover what it does, how to run it, how to test it and how to propose a change without needing private instructions.