create requirements.txt

[davidschlangen]

At first, let's have one common one at the base level here, with comments after each package on which game requires it.

Eventually, we want to come to a situation where each game has its own requirements list, and this is automatically merged into the requirements of this repo... (Because it would be too annoying to have to go into each directory and make sure that each games' requirements are satisfied, if all you want to do is to run the whole benchmark.)