Add installation of gh client to installation instructions

[hannesdatta]

Background:

We're using the GitHub command line client in class, but I haven't made the use and setup explicit in the course instructions.

Deliverable:

Please add the installation of GH client to the installation instructions for this class - AND - at Tilburg Science Hub (we have a building block on installing Git over there).

As a starter, here is code snippet for Mac: https://ports.macports.org/port/gh/.