Add link to sample-cli-plugin in developer guide (#466)

docs/DEVELOPER_GUIDE.md

@@ -29,7 +29,8 @@ Before creating a plugin, read the [Kubernetes Plugins documentation][plugins].
 **If you are writing a plugin in Go:** Consider using the
 [cli-runtime](https://github.com/kubernetes/cli-runtime/) project which is
 designed to provide the same command-line arguments, kubeconfig parser,
-Kubernetes API REST client, and printing logic.
+Kubernetes API REST client, and printing logic. Look at [sample-cli-plugin](https://github.com/kubernetes/sample-cli-plugin)
+for an example of a kubectl plugin.
 
 **Unofficial Quickstart for Go:** For Go plugins, there's a
 [GitHub template repo](https://github.com/replicatedhq/krew-plugin-template)