docs: clarify local clone setup

[AkilanManivannanak]

Keep the title exactly as it is:

docs: clarify local clone setup

For the description box, paste only this:

## Summary

Clarifies local development setup by noting that the repository root is not directly installable with `pip install -e .`.

The Python package source lives under `python-sdk`, so contributors should add that directory to `PYTHONPATH` when working from a local clone.

## Motivation

While setting up the repository locally, running `pip install -e .` from the repository root failed because neither `setup.py` nor `pyproject.toml` exists at the root.

This note should help future contributors avoid the same setup confusion.

## Testing

Verified that Python loads the local source with:

```bash
python -c "import nuscenes; print(nuscenes.__file__)"

[AkilanManivannanak]

Thanks for reviewing. I ran into this while setting up the repository locally and added a short clarification to help future contributors avoid the same confusion.