docs: Add CII Best Practices badge to README
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# actions-runner-controller (ARC)
+[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/6061/badge)](https://bestpractices.coreinfrastructure.org/projects/6061)
[![awesome-runners](https://img.shields.io/badge/listed%20on-awesome--runners-blue.svg)](https://github.com/jonico/awesome-runners)
This controller operates self-hosted runners for GitHub Actions on your Kubernetes cluster.
