Branching and tagging policy for releases (#519)
Branching and tagging policy for releases (#519)

* This commit:

1.  adds a branching policy for release branches,
2.  adds a tagging policy for release candidates, and
3.  adds a tagging policy for releases.

Related to

* releasing.md:  incorporate feedback from review

kubeflow/kubeflow,kubeflow/kubeflow,kubeflow/kubeflow,kubeflow/kubeflow,kubeflow/kubeflow,kubeflow/kubeflow