Add a boilerplate CONTRIBUTING.md document

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CONTRIBUTING.md

+## Contributing
+
+Thank you for your interest in contributing to this GitLab project! We welcome
+all contributions. By participating in this project, you agree to abide by the
+[code of conduct](#code-of-conduct).
+
+## Contributor license agreement
+
+By submitting code as an individual you agree to the [individual contributor
+license agreement][individual-agreement].
+
+By submitting code as an entity you agree to the [corporate contributor license
+agreement][corporate-agreement].
+
+## 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 can be
+reported by emailing contact@gitlab.com.
+
+This Code of Conduct is adapted from the [Contributor Covenant][contributor-covenant], version 1.1.0,
+available at [http://contributor-covenant.org/version/1/1/0/](http://contributor-covenant.org/version/1/1/0/).
+
+[contributor-covenant]: http://contributor-covenant.org
+[individual-agreement]: https://docs.gitlab.com/ee/legal/individual_contributor_license_agreement.html
+[corporate-agreement]: https://docs.gitlab.com/ee/legal/corporate_contributor_license_agreement.html