Add Mozilla code of conduct. [skip ci]

CODE_OF_CONDUCT.md

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+## Mozilla Community Participation Guidelines
+
+This repository is governed by Mozilla's code of conduct and etiquette guidelines.
+For more details, please read the
+[Mozilla Community Participation Guidelines](https://www.mozilla.org/about/governance/policies/participation/).
+
+## How to Report
+For more information on how to report violations of the CPG, please read our '[How to Report](https://www.mozilla.org/en-US/about/governance/policies/participation/reporting/)' page.
+
+## Project Specific Etiquette
+
+### Our Responsibilities
+
+Project maintainers are responsible for clarifying the standards of acceptable
+behavior and are expected to take appropriate and fair corrective action in
+response to any instances of unacceptable behavior.
+
+Project maintainers have the right and responsibility to remove, edit, or
+reject comments, commits, code, wiki edits, issues, and other contributions
+that are not aligned to this Code of Conduct, or to ban temporarily or
+permanently any contributor for other behaviors that they deem inappropriate,
+threatening, offensive, or harmful.
+
+Project maintainers who do not follow or enforce Mozilla's Participation Guidelines in good
+faith may face temporary or permanent repercussions.