Readme links to 404 of custom metrics API.

[Jandev]

The readme contains a link referencing to a 404, probably an old link:

A brief walkthrough what the statements mean:

horizontal-pod-autoscaler-use-rest-clients: "true" tells the controller manager to look for the custom metrics API

It's linking to https://github.com/kubernetes/community/blob/master/contributors/design-proposals/custom-metrics-api.md, which doesn't exist anymore.

I think the new link should be https://github.com/kubernetes/community/blob/master/contributors/design-proposals/instrumentation/custom-metrics-api.md, with the instrumentation folder added in the link.

[Jandev]

Nevermind, I see there's an active PR #41 addressing this issue already.