py: show Kubeflow alpha banner like website

[tarilabs]

Description

Kubeflow Model Registry is an Alpha component, per website: https://www.kubeflow.org/docs/components/model-registry/overview/#:~:text=Kubeflow%20Model%20Registry-,Alpha,-This%20Kubeflow%20component

This adds the same banner on the readme for the python wheel/library.

Hopefully this could be also propaedeutic to simply have one of the next py version to drop the .a1 suffix, I don't find it very ergonomic to access from pypi too.
@isinyaaa wdyt?

How Has This Been Tested?

Merge criteria:

• All the commits have been signed-off (To pass the DCO check)

• The commits have meaningful messages; the author will squash them after approval or in case of manual merges will ask to merge with squash.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work.
• Code changes follow the kubeflow contribution guidelines.

If you have UI changes

• The developer has added tests or explained why testing cannot be added.
• Included any necessary screenshots or gifs if it was a UI change.
• Verify that UI/UX changes conform the UX guidelines for Kubeflow.

[isinyaaa]

LGTM, thanks Matteo :)

[rareddy]

/lgtm
/approve

[google-oss-prow]

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This pull-request has been approved by: isinyaaa, rareddy

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