The guidelines should contain:

• What should I know before I get started?
  • Mandatory stuff
• How can i contribute?
  • Reporting Bugs
  • Before submitting a bug
  • How Do I Submit A (Good) Bug Report?
  • Suggesting Enhancements
• Pull Requests
  • Review Rules
• Styleguides
  • Git Commit Messages
  • Documentation Styleguide
• Additional Notes
  • Issue and Pull Request Labels
  • Explanation of Label "Prio"

The goal is to have some rules to save time, to ensure a good start in the project, build a better community with a friendly environment and let people want to become part of this project.

Example:
https://help.github.com/en/github/building-a-strong-community/setting-guidelines-for-repository-contributors
https://github.com/atom/atom/blob/master/CONTRIBUTING.md#how-can-i-contribute