Add a 'first contribution' walkthrough to CONTRIBUTING.md

[chillwur]

Description

CONTRIBUTING.md describes the workflow abstractly; a concrete worked example using a real (already-closed) good-first-issue would make the process much more approachable.

Deliverables

• A section in CONTRIBUTING.md walking through picking up an issue, forking, branching, writing a test, and opening a PR, using one of this backlog's simpler issues as the running example.

Depends on

None