docs: add CODE_OF_CONDUCT.md

Author: Mukller

CODE_OF_CONDUCT.md

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+# Contributor Covenant Code of Conduct
+
+## Our Pledge
+
+We as members, contributors, and leaders pledge to make participation in our community a harassment-free experience for everyone.
+
+## Our Standards
+
+Examples of behavior that contributes to a positive environment:
+- Using welcoming and inclusive language
+- Being respectful of differing viewpoints
+- Gracefully accepting constructive criticism
+- Focusing on what is best for the community
+
+Examples of unacceptable behavior:
+- Harassment of any kind
+- Trolling, insulting or derogatory comments
+- Public or private harassment
+- Publishing others' private information without permission
+
+## Enforcement
+
+Instances of abusive, harassing, or otherwise unacceptable behavior may be reported to the project maintainers.
+
+## Attribution
+
+This Code of Conduct is adapted from the [Contributor Covenant](https://www.contributor-covenant.org), version 2.1.
+
+---
+*[Mukller](https://github.com/Mukller)*