Add contributions guidelines

[AFulgens]

Context

CONTRIBUTING.md is missing from the project. Create it.

Resources

Some examples:

• https://github.com/rails/rails/blob/main/CONTRIBUTING.md
• https://github.com/helm/helm/blob/main/CONTRIBUTING.md
• https://github.com/opengovernment/opengovernment/blob/master/CONTRIBUTING.md
• https://github.com/cncf/foundation/blob/main/code-of-conduct.md