Update CONTRIBUTING.md

[michelle192837]

• Remove reference to Gubernator, which is now turned down
• Include additional guides on testing (as recommended in https://github.com/kubernetes/community/pull/7794).

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: michelle192837

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[ameukam]

/lgtm