Add "homework" UNIX exercises

[sjspielman]

As extra practice for the workshop attendees, we can include another directory in the rrp-workshop-exercises repository that contains stand-alone exercises to practice navigating their computer with fundamental UNIX commands as well as determining paths. These materials would not be covered in live training but would serve as extra resources for the attendees to work with on their own time, per their own interest level and need.

This content would be adapted, in part, from materials linked in #7