Add CITATION, CONTRIBUTING, and LICENSE files to every repo

[gvwilson]

Every project should include:

• https://github.com/datacamp/courses-intro-to-unix-shell/blob/master/CITATION.md
• https://github.com/datacamp/courses-intro-to-unix-shell/blob/master/CONTRIBUTING.md
• https://github.com/datacamp/courses-intro-to-unix-shell/blob/master/LICENSE.md

in its root directory (with appropriate edits to author names and project URLs).

[gvwilson]

See https://github.com/datacamp/python-course-template/pull/4 and https://github.com/datacamp/r-course-template/pull/10