Update contribution guidelines

[purva-thakre]

Fixes #150

When pip install -e. is used, the dependencies listed in pyproject.toml are not installed. I had to manually install them because I couldn't figure out a way to do something like pip install dev_dependencies. A line for this is currently missing.

Something to keep track of is black and flake8 will flag quite a few formatting issues when I run black toqito/ and flake8 toqito/.

[vprusso]

This is great. Thank you again for the contribution, @purva-thakre !