MAINT: Added Code of Conduct to .github folder (pandas-dev#19098)

https://help.github.com/articles/adding-a-code-of-conduct-to-your-project/

Github treats this file special in a few places. I think
https://github.com/pandas-dev/pandas-governance/blob/master/code-of-conduct.md
should always be considered the "official" home. Perhaps we could add
a note to the one added here about that.

[ci-skip]

.github/CODE_OF_CONDUCT.md

@@ -0,0 +1,63 @@
+# Contributor Code of Conduct
+
+As contributors and maintainers of this project, and in the interest of
+fostering an open and welcoming community, we pledge to respect all people who
+contribute through reporting issues, posting feature requests, updating
+documentation, submitting pull requests or patches, and other activities.
+
+We are committed to making participation in this project a harassment-free
+experience for everyone, regardless of level of experience, gender, gender
+identity and expression, sexual orientation, disability, personal appearance,
+body size, race, ethnicity, age, religion, or nationality.
+
+Examples of unacceptable behavior by participants include:
+
+* The use of sexualized language or imagery
+* Personal attacks
+* Trolling or insulting/derogatory comments
+* Public or private harassment
+* Publishing other's private information, such as physical or electronic
+  addresses, without explicit permission
+* Other unethical or unprofessional conduct
+
+Project maintainers have the right and responsibility to remove, edit, or
+reject comments, commits, code, wiki edits, issues, and other contributions
+that are not aligned to this Code of Conduct, or to ban temporarily or
+permanently any contributor for other behaviors that they deem inappropriate,
+threatening, offensive, or harmful.
+
+By adopting this Code of Conduct, project maintainers commit themselves to
+fairly and consistently applying these principles to every aspect of managing
+this project. Project maintainers who do not follow or enforce the Code of
+Conduct may be permanently removed from the project team.
+
+This Code of Conduct applies both within project spaces and in public spaces
+when an individual is representing the project or its community.
+
+A working group of community members is committed to promptly addressing any
+reported issues. The working group is made up of pandas contributors and users.
+Instances of abusive, harassing, or otherwise unacceptable behavior may be
+reported by contacting the working group by e-mail ([email protected]).
+Messages sent to this e-mail address will not be publicly visible but only to
+the working group members. The working group currently includes
+
+- Safia Abdalla
+- Tom Augspurger
+- Joris Van den Bossche
+- Camille Scott
+- Nathaniel Smith
+
+All complaints will be reviewed and investigated and will result in a response
+that is deemed necessary and appropriate to the circumstances. Maintainers are
+obligated to maintain confidentiality with regard to the reporter of an
+incident.
+
+This Code of Conduct is adapted from the [Contributor Covenant][homepage],
+version 1.3.0, available at
+[http://contributor-covenant.org/version/1/3/0/][version],
+and the [Swift Code of Conduct][swift].
+
+[homepage]: http://contributor-covenant.org
+[version]: http://contributor-covenant.org/version/1/3/0/
+[swift]: https://swift.org/community/#code-of-conduct
+