Add code of conduct

Ratified 2022-06-01.

CODE_OF_CONDUCT.md

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+# Code of Conduct
+
+## Our Pledge
+
+We as members, contributors, and leaders pledge to act and interact in ways that
+contribute to an open, welcoming and healthy community.
+
+## Our Principles
+
+### Assume good faith
+
+Contributors have many ways of reaching our common goals which may differ from
+your ways. Give others the chance to demonstrate that they are working towards
+those shared goals and are not trying to offend you.
+
+### Be respectful
+
+In order to assume good faith, we must also act in good faith. Be empathetic and
+kind toward other people, and be respectful of differing viewpoints, experiences, 
+and cultural backgrounds.
+
+### Be collaborative
+
+Give and gracefully accept constructive feedback, assistance, advice or
+mentorship. Be welcoming towards anyone who wishes to contribute.
+
+### Be open
+
+Preferably use public methods of communication unless posting something
+sensitive.
+
+## Moderation
+
+Community leaders are responsible for clarifying our principles of behavior and
+will take appropriate and fair corrective action in response to any behavior
+that they deem inappropriate, threatening, or harmful.
+
+Community leaders have the right to remove, edit, or reject comments, commits,
+code, wiki edits, issues, and other contributions that are not aligned to this
+Code of Conduct, and will communicate reasons for moderation decisions when
+appropriate.
+
+## Scope
+
+This Code of Conduct applies within all community spaces and also applies when
+an individual is officially representing the community in public spaces.
+Examples of representing our community include using an official e-mail
+address, posting via an official social media account, or acting as an
+appointed representative at an online or offline event.
+
+## Attribution
+
+This Code of Conduct is inspired by and partially adapted from the [Debian Code
+of Conduct](https://www.debian.org/code_of_conduct), version 1.0 and the
+[Contributor Covenant](https://www.contributor-covenant.org/version/2/1/code_of_conduct.html),
+version 2.1.