Pulled over a code-of-conduct.md from a random Arduino library (the
CircuitPython one mentions a lot of CircuitPython specifics), and added
**Contributing** section to README. Curious that there isn't a central
place where a code of conduct can be stored, with a reference rather
than a copy in each repo.
Updated CHANGELOG for 1.3.1, 1.4.0, and 1.5.0.
