A lot of open source projects carry guidance on how to contribute to their projects effectively, which is incredibly useful for newcomers to the project who wish to help and need to get started. The current documentation does have a contribution page but could go into a lot more detail.
From a development perspective at least I can think of:
- Which issues are able to be worked on, and who decides that?
- Basic process instructions - assigning issues, changing to correct state in the board, etc
- Submitting pull requests - what to check before submitting, the management process behind the PR and related issue(s), review process etc
Also, these instruction may appear on the documentation site, but a developer's first port of call will typically be the README of a repository, so they should at least be directed to the documentation page.
Some examples of contribution guidelines (though there are many) are:
- Angular
- Groovy