Install/use rendergit with uv

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

```sh
uv tool install git+https://github.com/karpathy/rendergit
```
Then `rendergit` will be added to your path.

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

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