add: uv instructions

[metaist]

I like using the built-ins, but perhaps I could document the faster path.

[metaist]

Going to skip this for now:

• It's another obstacle for someone to get started quickly.
• It's another dependency (even though they're super fast and good).
• If you know how to use rye and uv, the instructions in CONTRIBUTING.md won't phase you: pip install becomes uv pip install

[metaist]

With the release of uv 0.3.0, it makes more sense to include these instructions.