Add Deprecation Notice to README of openmpi package

[everpeace]

As discussed in #778 (comment), we will eventually remove openmpi package at 0.5.0 (#1859).

I put a deprecation notice to README of openmpi package to resolve the issue (kubeflow/website#272)

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This change is 

[everpeace]

/assign @jlewi
Please take a look.

[alsrgv]

LGTM, MPI operator is great!
/approve

[jlewi]

Thanks!
/test all
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: alsrgv, jlewi

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[everpeace]

/retest

[jlewi]

Test failures:
#2142 test_runner complaining about improper usage; is it a flake?
kubeflow/testing#229 - minikube shouldn't have been triggered

/test all

[jlewi]

/test all

[jlewi]

I submitted a fix for improper triggering of tests (kubeflow/testing#229).
Hopefully that fixes the test failures.

/test all

[jlewi]

minikube test wasn't triggered but it looks like the studyjobcontroller deploy test failed.

[jlewi]

/test all

[jlewi]

If the tests don't pass I'll merge this manually since its just a docs change.

[k8s-ci-robot]

@everpeace: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
kubeflow-presubmit	caa2842	link	/test kubeflow-presubmit

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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