Adding support for pip in README.md

[asquare14]

Presently only conda is mentioned. We could generate a requirements.txt or environment.txt file and let users setup the project using pip install after creating a virtual environment. This is especially helpful for Linux users.

[birdsarah]

If you feel motivated to that's fine. Unless other core team feel differently, I strongly prefer keeping conda as the default as it removes cross platform support headaches.