Add a `CODE_OF_CONDUCT.md`.

[miya0001]

• https://make.wordpress.org/community/handbook/wordcamp-organizer/planning-details/code-of-conduct/
• https://help.github.com/articles/adding-a-code-of-conduct-to-your-project/

[danielbachhuber]

Just a note on this: I talked with Andrea Middleton about codes of conduct this weekend at WCUS. It sounds like this is being considered at the project level. The big challenge to overcome is "how do you enforce it?"