add code of conduct

CODE_OF_CONDUCT.md

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+International law supersedes all codes of conduct, including this one.
+Authors of codes of conduct presuming the authority to create new laws
+should be brought to justice.
+
+This being said, if you still insist on wringing advice from us:
+Be kind. Be professional. Be friendly. Be respectful. Focus on the project.
+[Report inappropriate conduct](https://help.github.com/en/articles/reporting-abuse-or-spam).
+
+Contributions which fail to conform to these guidelines may be reworded,
+censored, and/or reported as abuse without notice.
+
+## The long explanation
+
+We celebrate the fact that all contributions come from different humans with
+their own unique attributes, background and history. However we also place a
+strong emphasis on neutrality and a simple focus on the project's goals.
+Contributions are thus particularly welcome when references to gender,
+ethnicity, religion and so on are avoided as much as possible.
+This is for inclusiveness, professionality, and also privacy and security.
+This likely means that apart from usernames (which frequently unavoidably hint
+about the author's background) there should be no personal information included
+in contributions.
+
+Note that we have no contract with contributors. Introducing contributor licence
+agreements (CLAs) would allow us to enforce additional rules but would also be a
+scary barrier for new contributors. We don't want to be scary. We love you!