Create CODE_OF_CONDUCT.md

[rmzelle]

Per #4471. Probably should float this past https://discourse.citationstyles.org/. Used the GitHub template per https://help.github.com/en/github/building-a-strong-community/adding-a-code-of-conduct-to-your-project with https://citationstyles.org/contact/ as contact channel.

[csl-bot]

Awesome! You just created a pull request to the Citation Styles Language styles repository. One of our human volunteers will try to get in touch soon (usually within a week). In the meantime, I will run some automated checks. You should be notified of the results in a few minutes.

If you haven't done so yet, please make sure your style validates and follows all our other Style Requirements.

To update this pull request, visit the "Files changed" tab above, and click on the pencil icon (see below) in the top-right corner of your style to start editing.

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[csl-bot]

😟 There are some issues with your submission.

1 test failed

The CSL Style Repository may not contain extra files (make sure styles have a ".csl" extension)

expected: []
     got: ["/home/travis/build/citation-style-language/styles/CODE_OF_CONDUCT.md"]

(compared using ==)

Please check the test report for details.

[csl-bot]

😃 Your submission passed all our automated tests.

[rmzelle]

Apparently GitHub also has default community health files (https://help.github.com/en/github/building-a-strong-community/creating-a-default-community-health-file) so we don't have to populate each repo individually.