Add code of conduct

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

CODE_OF_CONDUCT.md

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+<!--
+SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+
+SPDX-License-Identifier: CC-BY-SA-4.0
+-->
+
+Contributor Code of Conduct
+===
+
+As contributors and maintainers of this project, and in the interest of fostering an open and
+welcoming community, we pledge to respect all people who contribute through reporting issues,
+posting feature requests, updating documentation, submitting pull requests or patches, and other
+activities.
+
+We are committed to making participation in this project a harassment-free experience for everyone,
+regardless of level of experience, gender, gender identity and expression, sexual orientation,
+disability, personal appearance, body size, race, ethnicity, age, religion, or nationality.
+
+Examples of unacceptable behavior by participants include:
+
+* The use of sexualized language or imagery
+* Personal attacks
+* Trolling or insulting/derogatory comments
+* Public or private harassment
+* Publishing other's private information, such as physical or electronic addresses, without explicit
+  permission
+* Other unethical or unprofessional conduct.
+
+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. By
+adopting this Code of Conduct, project maintainers commit themselves to fairly and consistently
+applying these principles to every aspect of managing this project. Project maintainers who do not
+follow or enforce the Code of Conduct may be permanently removed from the project team.
+
+This code of conduct applies both within project spaces and in public spaces when an individual is
+representing the project or its community.
+
+Instances of abusive, harassing, or otherwise unacceptable behavior may be reported by opening an
+issue or contacting one or more of the project maintainers.
+
+This Code of Conduct is adapted from the [Contributor Covenant](http://contributor-covenant.org),
+version 1.2.0, available at
+[http://contributor-covenant.org/version/1/2/0/](http://contributor-covenant.org/version/1/2/0/)