Add CONTRIBUTING.md

What does this MR do?

We should document how to contribute to this project, and refer users to the main issue tracker so that they can best be assisted in their contributions, and prioritization.

What are the relevant issue numbers?

Closes #6 #2 #1

(These should not be open)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for this project, if necessary
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Security reports checked/validated by reviewer