Docs Request: PyPI addition

[jamesbraza]

Before I begin, I think a compliment for this repo is that Stanford University is actually using this in its graduate-level CS234 Reinforcement Learning homeworks! 🥳

Students are being instructed to manually git clone and pip install this repo locally. At first, I posed directly adding the GitHub repo to requirements.txt like so:

git+ssh://git@github.com/kenjyoung/MinAtar.git

I decided to take a look at PyPI, and saw MinAtar is already published there.

The request is: to update the README.md to instruct to users to install from PyPI, unless they're going to be contributing.

[kenjyoung]

Thanks! I added a note about this to the README as well as a note that the gym interface has recently changed.

[jamesbraza]

Sounds good, thank you! Case closed.