Adds kubectl and kind/minikube requirement to README (#266)

Signed-off-by: Paul S. Schweigert <[email protected]>

Author: psschwei

README.md

@@ -6,6 +6,8 @@
 
 ## Getting Started
 
+Note: In order to use the `quickstart` plugin, you must install the [Kubernetes CLI `kubectl`](https://kubernetes.io/docs/tasks/tools/install-kubectl) and either [`kind`](https://kind.sigs.k8s.io/docs/user/quick-start) or [`minikube`](https://minikube.sigs.k8s.io/docs/start/).
+
 ### Installation
 
 You can download the latest binaries from the [Releases](https://github.com/knative-sandbox/kn-plugin-quickstart/releases) page.