Add CONTRIBUTING guide

[philprime]

Many Open Source projects on GitHub use a special file called CONTRIBUTING.md to describe how the contribution process is defined. This might need to be written according to grants.

Docs:

• https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors
• https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/creating-a-default-community-health-file

Examples:

• https://github.com/Alamofire/Alamofire/blob/master/CONTRIBUTING.md
• https://github.com/yonaskolb/XcodeGen/blob/master/CONTRIBUTING.md