Add kubectl datadog plugin doc

docs/getting_started.md

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 ```
 
+## Install the kubectl plugin
+
+[kubctl plugin doc](/docs/kubectl-plugin.md)
+
 ## Cleanup
 
 The following command deletes all the Kubernetes resources created by the Datadog Operator and the linked `DatadogAgent` `datadog`.

docs/kubectl-plugin.md

@@ -0,0 +1,64 @@
+# Datadog Operator kubectl plugin
+
+## Install the plugin with krew
+
+The Datatadog Operator comes with a kubectl plugin providing a set of helper utilities giving visibility into certain internal components which some users may find useful.
+
+To install please use the [krew plugin manager](https://krew.sigs.k8s.io/)
+
+The krew plugin manifest url can be found on the project [release page](https://github.com/DataDog/datadog-operator/releases). Each release has its own `datadog-plugin.yaml` manifest file.
+
+```console
+$ kubectl krew install --manifest-url https://github.com/DataDog/datadog-operator/releases/download/<release-version>/datadog-plugin.yaml
+Installing plugin: datadog
+Installed plugin: datadog
+\
+ | Use this plugin:
+ | 	kubectl datadog
+ | Documentation:
+ | 	https://github.com/DataDog/datadog-operator
+/
+```
+
+## Available commands
+
+```console
+$ kubectl datadog --help
+Usage:
+  datadog [command]
+
+Available Commands:
+  agent
+  clusteragent
+  flare        Collect a Datadog's Operator flare and send it to Datadog
+  get          Get DatadogAgent deployment(s)
+  help         Help about any command
+  validate
+
+```
+
+### Agent sub-commands
+
+```console
+$ kubectl datadog agent --help
+Usage:
+  datadog agent [command]
+
+Available Commands:
+  check       Find check errors
+  find        Find datadog agent pod monitoring a given pod
+  upgrade     Upgrade the Datadog Agent version
+
+```
+
+### Cluster Agent sub-commands
+
+```console
+$ kubectl datadog clusteragent --help
+Usage:
+  datadog clusteragent [command]
+
+Available Commands:
+  leader      Get Datadog Cluster Agent leader
+  upgrade     Upgrade the Datadog Cluster Agent version
+```