Merge pull request #530 from Unity-Technologies/docs-installation-python

update to python instructions

docs/Installation.md

 
 ### Mac and Unix Users
 
+[Download](https://www.python.org/downloads/) and install Python 3 if you do not already have it.
+
 If your Python environment doesn't include `pip`, see these 
 [instructions](https://packaging.python.org/guides/installing-using-linux-tools/#installing-pip-setuptools-wheel-with-linux-package-managers)
 on installing it.