Update documentation on how to add koans to an existing project

[wcravens]

Hint: This works in project.clj

  :dev-dependencies [[lein-koan "0.1.4"]]
  :plugins [[lein-koan "0.1.4"]]
  :profiles { :dev {:dependencies [[lein-koan "0.1.4"]]}}

Also document the need for resources/koans.clj to exist.