Pull request exercise with our book

[billspat]

write up an in-class exercise for participants to fork this book/repository and add examples to our examples. Need to add category of example

• package
• project
• reference

Prior to the exercise, walk through it once as a group. Someone volunteers an example repo, we then ask the group to categorize and why, then do a fork and a PR in our personal github accounts with the example.

can github to edit the markdown directly rather than get book down running

advanced user: provide a PR story for how they were able to contribute to a project, or an interaction they had on gitub.