Add instructions to generate `requirements.in` with conda?

[rkurchin]

I really appreciate the instructions and discussion in the environment section (and learned some things myself from it!). My personal default is still to manage my local environments in conda, so in the very envisionable scenario that I already had a conda environment I'd been using for the research that I'm now putting into one of our templates, it might make sense to include instructions to generate the requirements.in file using conda.

I think as long as you have pip in your conda environment, this is as simple as pip freeze > requirements.in. This probably results in some "overpinning" of versions (which could obviously be manually changed if desired), but I think it would work?