Document python command in different environments

I realized this evening that multiple Python versions installed on Windows coexist nicely as documented here: https://docs.python.org/3/using/windows.html

For this workshop, using 3.4 installed through the official installation package is done by using `py -3` instead of `python.exe`.

I know for Ubuntu, `python` usually indicates 2.7 usage while `python3` is linked to the latest version of 3.
