Forbid LLM contributions (#1941)

Co-authored-by: Christophe Coevoet <stof@notk.org>

CONTRIBUTING.md

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+See [the README](README.md) for details on how to write and run tests in this
+repository.
+
+## Large Language Models
+
+Do not submit any code or prose written or modified by large language models or
+"artificial intelligence" such as GitHub Copilot or ChatGPT to this project.
+These tools produce code that looks plausible, which means that not only is it
+likely to contain bugs those bugs are likely to be difficult to notice on
+review. In addition, because these models were trained indiscriminately and
+non-consensually on open-source code with a variety of licenses, it's not
+obvious that we have the moral or legal right to redistribute code they
+generate.