Add CoC

[sdboyer]

Repo needs a code of conduct.

[metalmatze]

Examples:
https://github.com/prometheus/prometheus/blob/main/CODE_OF_CONDUCT.md
https://github.com/thanos-io/thanos/blob/main/CODE_OF_CONDUCT.md

[RichiH]

CNCF CoC is a decent default.

[carolynvs]

The CNCF Code of Conduct is the Contributor Covenant 1.2.