Explain how to get started contributing in contributing.md

[Powersource]

I think it at least should mention

• that Good first issues are a thing
• that you can ask for pair programming/mentoring to get started. Where should we direct them to do this? To a specific person (that could maybe then delegate?)?

[stale]

Is this still relevant? If so, what is blocking it? Is there anything you can do to help move it forward?