Add A CONTRIBUTING.md File

[randomPoison]

We already have mentored tasks so it would be good to have a guide to help new contributors get started, especially since the development process isn't well documented at this point.

Things to cover:

• How to get started (meaning compile the engine and run the examples).
• Explain the issue labels so that people can find tasks that suit them.
• Explain mentored issues.
• Explain the pull request and contribution process.
• Add a code of conduct.

See the rust-clippy repo for an example.