docs: add a code of conduct

CODE_OF_CONDUCT.md

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+# Code of conduct
+
+Catchlight is maintained by one person. The aim is straightforward. Anyone should be able to
+file a bug, ask a question, or disagree with a decision here without it costing them
+anything.
+
+## What's expected
+
+Be straight with people. Assume the person on the other end is trying to help, or trying to
+get help, and not much else. Criticism of code, decisions and documentation is welcome, and
+it is most of what happens here. Criticism of people is not.
+
+## What isn't on
+
+- Harassment of any kind, including sustained unwanted contact
+- Personal attacks, or remarks about who someone is rather than what they argued
+- Publishing anyone's private information
+- Deliberately derailing threads or wasting people's time
+
+## What happens if it goes wrong
+
+I read everything that comes in. If something crosses the line I will edit or delete it and
+say why. If it carries on I will block the account. There is no committee and no appeals
+process, because there is one of me. I would rather say that plainly than point you at a
+procedure that doesn't exist.
+
+## Where this applies
+
+Issues, pull requests and discussions in this repository. If something happens elsewhere and
+it affects whether people feel safe taking part here, I'll take that into account.
+
+## Contact
+
+security@considus.com. Security problems have their own private path in
+[SECURITY.md](SECURITY.md), and reports of those should go there rather than into a public
+issue.