Adds in a Code of conduct

Gutenberg came under the WordPress etiquette page but it is a good idea to explicitly call it out in the project.

Author: Tammie Lister

CODE_OF_CONDUCT.md

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+This project comes under the WordPress [Etiquette](https://wordpress.org/about/etiquette/):
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+In the WordPress open source project, we realize that our biggest asset is the community that we foster. The project, as a whole, follows these basic philosophical principles from The Cathedral and The Bazaar.
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+- Contributions to the WordPress open source project are for the benefit of the WordPress community as a whole, not specific businesses or individuals. All actions taken as a contributor should be made with the best interests of the community in mind.
+- Participation in the WordPress open source project is open to all who wish to join, regardless of ability, skill, financial status, or any other criteria.
+- The WordPress open source project is a volunteer-run community. Even in cases where contributors are sponsored by companies, that time is donated for the benefit of the entire open source community.
+- Any member of the community can donate their time and contribute to the project in any form including design, code, documentation, community building, etc. For more information, go to make.wordpress.org.
+- The WordPress open source community cares about diversity. We strive to maintain a welcoming environment where everyone can feel included, by keeping communication free of discrimination, incitement to violence, promotion of hate, and unwelcoming behavior.
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+The team involved will block any user who causes any breach in this.