code-of-conduct.md: Add CoC

[ashcrow]

Based off https://github.com/openshift/cluster-monitoring-operator/blob/master/code-of-conduct.md

Resolved #144

/cc @lucab @smarterclayton

[openshift-ci-robot]

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[ashcrow]

/cc @steveej @crawford

[lucab]

I think there is a need for a proper CoC covering OpenShift (+COS) projects.

Major problems with the one in this PR:

• it is missing any contact points
• it is missing a designed (and trained) responsible to mediate issues
• it reads like a quick sed-replace of the original CoreOS one
• there is no such a thing like "cincinnati events" and "cincinnati entity"

Instead, we'd like to have an umbrella CoC and just point to it, like https://github.com/prometheus/prometheus/blob/master/code-of-conduct.md.

[ashcrow]

/hold