📖 add kubebuilder workshop in the book

Author: Mengqi Yu

docs/book/README.md

@@ -41,6 +41,7 @@ Including:
 
 GitBook: [book.kubebuilder.io](http://book.kubebuilder.io)<br/>
 GitHub Repo: [kubernetes-sigs/kubebuilder](https://github.com/kubernetes-sigs/kubebuilder)<br/>
+Workshop: [kubebuilder-workshop](https://github.com/pwittrock/kubebuilder-workshop)<br/>
 Slack channel: [#kubeuilder](http://slack.k8s.io/#kubebuilder)<br/>
 Google Group: [[email protected]](https://groups.google.com/forum/#!forum/kubebuilder)<br/>
 Planned Work: [Sprint Dashboard](https://github.com/kubernetes-sigs/kubebuilder/projects/1)<br/>