Sometimes, a repository will have a "contributing file" like this one. You'll usually see this if
- The repository has a LOT of code
- A LOT of people want to contribute to the repository
- The repository is accepting contributions from other people
Some small examples include:
- dotfiles.github.com
- awesome-bash's contributing.md
- tj/git-extras's CONTRIBUTING.md
- schemastore's CONTRIBUTING.md
If you are interested in contributing to other (open source) projects, this article on opensource.guide is a very useful thorough guide/walkthrough/explainer.