Prepare for releasing v0.4.0

[tenzen-y]

Mainly, I updated the release note.

Note: Blocked on #540 #543.

/assign @alculquicondor @terrytangyuan

[alculquicondor]

/hold (to merge at the right time)

[google-oss-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

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[alculquicondor]

/lgtm

[tenzen-y]

@alculquicondor Once #545 is merged into the master branch, can we merge this PR?

[alculquicondor]

Let me try to upload the images first.

[tenzen-y]

Let me try to upload the images first.

Sure.

[tenzen-y]

Oh, conflicts are happening.

[tenzen-y]

Oh, conflicts are happening.

Resolved.

[alculquicondor]

/lgtm

we can merge this right after the release is published in github

[alculquicondor]

or actually, before should be good

/hold cancel

[tenzen-y]

or actually, before should be good

Previously (v0.3.0), we merged before cutting a new release.

https://github.com/kubeflow/mpi-operator/commits/v0.3.0