Add Code of Conduct to .github folder. (GH-4029)

.github/CODE_OF_CONDUCT.rst

+Code of Conduct
+===============
+
+Please note that all interactions on
+`Python Software Foundation <https://www.python.org/psf-landing/>`__-supported
+infrastructure is `covered
+<https://www.python.org/psf/records/board/minutes/2014-01-06/#management-of-the-psfs-web-properties>`__
+by the `PSF Code of Conduct <https://www.python.org/psf/codeofconduct/>`__,
+which includes all infrastructure used in the development of Python itself
+(e.g. mailing lists, issue trackers, GitHub, etc.).
+
+In general this means everyone is expected to be open, considerate, and
+respectful of others no matter what their position is within the project.
+