Add CII Best Practices badge to README.md
For more information about the badge see
<https://bestpractices.coreinfrastructure.org>. Information about
the kubeone badge is located here:
<https://bestpractices.coreinfrastructure.org/en/projects/2934>
<a href="https://goreportcard.com/report/github.com/kubermatic/kubeone">
<img src="https://goreportcard.com/badge/github.com/kubermatic/kubeone" alt="Go Report Card" />
</a>
+ <a href="https://bestpractices.coreinfrastructure.org/projects/2934"><img src="https://bestpractices.coreinfrastructure.org/projects/2934/badge">
+ </a>
</p>
`kubeone` is a CLI tool and a Go library for installing, managing, and upgrading
