Add code of conduct as suggested by Bundler

CODE_OF_CONDUCT.md

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+Contributor Code of Conduct
+
+As contributors and maintainers of this project, 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, or religion.
+
+Examples of unacceptable behavior by participants include the use of sexual
+language or imagery, derogatory comments or personal attacks, trolling, public
+or private harassment, insults, or other 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. Project maintainers who do not
+follow the Code of Conduct may be 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 1.1.0, available at
+http://contributor-covenant.org/version/1/1/0/