Write instructor notes for each lesson in _extras/guide.md

[Zena Lapp]

_extras/guide.md: https://github.com/UMSWC/curriculum/blob/gh-pages/_extras/guide.md

[Kelly Sovacool, PhD]

This is where it's not super ideal that we combined multiple lessons (r, unix, git) into one lesson repo. Definitely will need to make use of section headers and maybe html details tags to keep it neat.