Provide example on HPA usage from example

[Alb0t]

Example usage is great but I was looking at this specifically for using HPA and wanted to see if I could get it working with just the test-adapter first. HPA example added!

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[Alb0t]

/check-cla

[Alb0t]

/assign @kawych

[s-urbaniak]

Thanks a lot for the contribution! This indeed makes sense :+1: /lgtm

[s-urbaniak]

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Alb0t, s-urbaniak

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/custom-metrics-apiserver/blob/master/OWNERS)~~ [s-urbaniak] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment