Install/use rendergit with uv #6

It'd be nice to add to the readme how to use it as a standalone tool with uv like:

uv tool install git+https://github.com/karpathy/rendergit

Then rendergit will be added to your path.

rendergit https://github.com/karpathy/nanogpt

Using uv tool install: https://docs.astral.sh/uv/guides/tools/#installing-tools