Add note to readme about kubectl version (#190)

README.md

@@ -17,9 +17,10 @@ Quick troubleshooting for your Azure Kubernetes Service (AKS) cluster.
 3. [Compatibility](#compatibility)
 4. [Current Feature Set](#current-feature-set)
 5. [User Guide](#user-guide)
+   1. [Prerequisites](#prerequisites)
    1. [Raw Kustomize](#kustomize-deployment)
-   2. [Azure CLI Kollect Command](#using-azure-command-line-tool)
-   3. [VS Code AKS Extension](#using-vs-code-aks-extension)
+   1. [Azure CLI Kollect Command](#using-azure-command-line-tool)
+   1. [VS Code AKS Extension](#using-vs-code-aks-extension)
 6. [Programming Guide](#programming-guide)
    1. [Automated Tests](#automated-tests)
 7. [Dependent Consuming Tools and Working Contract](#dependent-consuming-tools-and-working-contract)
@@ -66,6 +67,10 @@ Periscope collects the following logs and metrics:
 
 You can deploy Periscope to your cluster in different ways, depending on your preferred working environment and the degree of control over precisely how it needs to be run.
 
+### Prerequisites
+
+Whichever method you choose to deploy Periscope, you will need to have [kubectl](https://kubernetes.io/docs/tasks/tools/#kubectl) **version 1.21** or later installed on your system.
+
 ### Kustomize Deployment
 
 This approach gives you the most control, allowing you to inspect and override the configuration of all Kubernetes resources deployed to your cluster, as well as choosing a specific release.