README: Add tf-operator v1alpha2 design doc (#553)
README: Add tf-operator v1alpha2 design doc (#553)

Signed-off-by: Ce Gao <1d82c7e569a19de560a5cb3a123e2745e610ad0f@caicloud.io>
kubeflow/training-operator,kubeflow/training-operator,kubeflow/training-operator