Add setup info for Python SDK

[shaowei-su]

This change enables easy command line pip installation for the Python SDK v1 & v2beta1, e.g

pip install git+https://github.com/kubeflow/mpi-operator.git#subdirectory=sdk/python/v2beta1

Ideally the package should be published to Pypi but skip it for now since mpi-operator v2 will eventually merged into training-operator and Python SDK is already maintained in the other repo (kubeflow/training-operator#1380).

[google-oss-prow]

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[alculquicondor]

we won't necessarily move the v2 operator to training

kubeflow/community#557 (open discussion)

Any python experts are able to review this?

[shaowei-su]

Closing for now since the the SDK seems lack basic client interface like TFJobClient (https://github.com/kubeflow/training-operator/blob/master/sdk/python/kubeflow/training/api/tf_job_client.py).