Description of proposed changes

Refreshes our code of conduct from v1.4 to v2.0, and also add a badge to the main README.md. Community discussion can take place at #672.

Changes (copied from https://github.com/ContributorCovenant/contributor_covenant/blob/29000ea9977e75b76189335f376a42a833972433/changelog#L19-L26):

• Emphasizes 'community' over 'project' scope, effectively merging the Community Covenant into the Contributor Covenant
• Adds expectation about accepting responsibility and apologizing to those affected by mistakes, and learning from the experience to the list
• Adds sexual attention and advances of any kind as unacceptable behavior example
• Moves enforcement responsibilities from project maintainers to community leaders
• Adds responsibility to communicate reasons for moderation decisions when removing, editing, or rejecting contributions not aligned to the Code of Conduct
• Requires community leaders to respect privacy and security of the reporter of any incident, not just confidentiality
• Provides a template for code of conduct enforcement

The text is copied over from https://www.contributor-covenant.org/version/2/0/code_of_conduct/. Feel free to tweak any of the points if there is anything you feel should be in (or not in) the document.

Fixes #672

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Notes

• You can write /format in the first line of a comment to lint the code automatically