add venv to contributing doc

[turboOrange]

Description of the problem

Trying to set up this code for development for the first time, I followed the instructions without thinking too much, and it didn't work. I got this message when running maturin develop:

💥 maturin failed Caused by: Couldn't find a virtualenv or conda environment, but you need one to use this command. For maturin to find your virtualenv you need to either set VIRTUAL_ENV (through activate), set CONDA_PREFIX (through conda activate) or have a virtualenv called .venv in the current or any parent folder. See https://virtualenv.pypa.io/en/latest/index.html on how to use virtualenv or usematurin buildandpip install <path/to/wheel>instead.

Solution

What was missing were the commands related to setting up the virtual environment.