Add code of conduct document

This makes several minor changes to the Contributor Convenant, v1.4,
which was provided by GitHub as a template.

CODE_OF_CONDUCT.md

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+# Code of Conduct (Contributor Covenant)
+
+For answers to common questions about this code of conduct, see
+https://www.contributor-covenant.org/faq
+
+## Our Pledge
+
+To foster an open and welcoming environment, we as maintainers and
+contributors pledge to make participation in our project and our
+community a harassment-free experience for everyone, regardless of
+age, body size, disability, ethnicity, sex characteristics, gender
+identity and expression, level of experience, education,
+socio-economic status, nationality, personal appearance, race,
+religion, or sexual identity and orientation.
+
+## Our Standards
+
+Examples of behavior that contributes to creating a positive environment
+include:
+
+* Using welcoming and inclusive language
+* Being respectful of differing viewpoints and experiences
+* Gracefully accepting constructive criticism
+* Focusing on what is best for the community
+* Showing empathy toward other community members
+
+Examples of unacceptable behavior by participants include:
+
+* The use of sexualized language or imagery and unwelcome sexual attention or
+ advances
+* Trolling, insulting/derogatory comments, and personal or political attacks
+* Public or private harassment
+* Publishing others' private information, such as a physical or electronic
+ address, without explicit permission
+* Other conduct which could reasonably be considered inappropriate in a
+ professional setting
+
+## Our Responsibilities
+
+Project maintainers are responsible for clarifying the standards of acceptable
+behavior and are expected to take appropriate and fair corrective action in
+response to any instances of unacceptable behavior.
+
+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 with this Code of Conduct, or to ban temporarily or
+permanently any contributor for other behavior that is inappropriate,
+threatening, offensive, or harmful.
+
+## Scope
+
+This Code of Conduct applies both within project spaces and in public spaces
+when an individual is representing the project or its community. Examples of
+representing a project or community include making an official online post
+about the project or acting as an appointed representative at an online or
+offline event. Representation of a project may be further defined and
+clarified by project maintainers.
+
+## Enforcement
+
+Instances of abusive, harassing, or otherwise unacceptable behavior
+may be reported by contacting the initial creator of CATCH and lead
+project maintainer at hayden@mit.edu. All complaints will be reviewed
+and investigated and will result in a response that is deemed
+necessary and appropriate to the circumstances. The project maintainers will
+maintain confidentiality with regard to the reporter of an incident. Further
+details of specific enforcement policies may be posted separately.
+
+Project maintainers who do not follow or enforce the Code of Conduct in good
+faith may face repercussions as determined by other members of the project's
+leadership.
+
+## Attribution
+
+This Code of Conduct is adapted from the [Contributor Covenant][homepage], version 1.4,
+available at https://www.contributor-covenant.org/version/1/4/code-of-conduct.html
+
+[homepage]: https://www.contributor-covenant.org