Add code of conduct

Author: js-choi

Diff for: CODE_OF_CONDUCT.md

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+# Code of conduct
+This repository is a TC39 project, and it therefore subscribes to its [code of
+conduct][CoC]. It is available at <https://tc39.es/code-of-conduct/>.
+
+We all should strive here to be respectful, friendly and patient, inclusive,
+considerate, and careful in the words we choose. When we disagree, we should try
+to understand why.
+
+To ask a question or report an issue, please follow the [CoC]’s directions,
+e.g., emailing [[email protected]][].
+
+More information about contributing is also available in [CONTRIBUTING.md][].
+
+[CoC]: https://tc39.es/code-of-conduct/
+[[email protected]]: mailto:[email protected]
+[CONTRIBUTING.md]: https://github.com/tc39/proposal-pipeline-operator/blob/main/CONTRIBUTING.md