Add Machine API Operator reference

[enxebre]

What this PR does / why we need it:
We are leveraging the Cluster API Project (particularly what refers to the machine API) in OpenShift for conveying desired stated of machines and building upper level tooling on top of it e.g autoscaler/node-remediation. This adds API Adoption reference to Machine API Operator

[k8s-ci-robot]

Hi @enxebre. Thanks for your PR.

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[roberthbailey]

/ok-to-test
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: enxebre, roberthbailey

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