Making installation/ramp-up easier.

[korymath]

Great work on the project! I wanted to add my notes for getting this system running on its own. I find that it can help to make explicit lots of the steps of installation to help avoid issues in the future.

This PR:

• [x] includes explicit details on setting up a virtual environment for running MinAtar/
• [x] adds a requirements.txt file to address: https://github.com/kenjyoung/MinAtar/issues/1
• [x] adds a basic python .gitignore to ignore virtual environment and other temporary and unneeded python files.
• [x] adds a line in the README on expected output following proper installation.

Thank you again for releasing the code!

[korymath]

Addressed your fixes, thank you for the quick turnaround.