update contributing

CONTRIBUTING.md

@@ -96,7 +96,8 @@ Here's how you can set up your local development environment to contribute.
       uv venv -p 3.12 --seed
       ```
    4. Activate it. On Linux, this is `. .venv/bin/activate`, on Windows `.\.venv\Scripts\activate`.
-2. Install Narwhals: `uv pip install -e ".[dev, docs]"`
+2. Install Narwhals: `uv pip install -e ".[dev, docs]"`. If you also want to test Dask , PySpark, and
+   Modin, you can install them too with `uv pip install -e ".[dev, docs, extra]"`.
 3. Install a fork of griffe:
    ```
    uv pip install git+https://github.com/MarcoGorelli/griffe.git@no-overloads