Move "list" example further down in README

* This improves the "flow" of the examples by showing the use of `list`
  at the point in the example where two users have been added.

[#14]
Add final `list` example to README

* Per the natural flow of the example:
  1. Add users
  2. List users
  3. Remove users
  4. List users

[#14]
Use slightly more strict \S for gh user regex

[#14]