Update contribution guidelines

[joelostblom]

I was setting up a plotly dev env again and documented a few things I did differently from the current guidelines. The only thing that I didn't include was that I had issues installing the optional dependencies via pip. This might be a local problem, but fwiw I installed these packages via conda instead:

conda create -n plotly-dev pandas seaborn jupyterlab scipy simplegeneric
conda activate plotly-dev
conda install -c plotly plotly-orca psutil requests

[emmanuelle]

Thanks @joelostblom. I think most people take a look at this info on github, so is it OK if we review/merge after 4.3?

[jonmmease]

Thanks @joelostblom! I'm going to merge this and then add a follow-up commit removing the submodules section now that these are all inlined (as of #1942)