doc: add imagePullSecrets in kubernetes default example

Author: pkoutsovasilis

deploy/helm/elastic-agent/examples/kubernetes-default/README.md

@@ -13,13 +13,30 @@ In this example we install the built-in `kubernetes` integration with the defaul
 2. `kubernetes` integration assets installed through Kibana ([Kibana - Install and uninstall Elastic Agent integration assets](https://www.elastic.co/guide/en/fleet/current/install-uninstall-integration-assets.html))
 
 ## Run:
+
+#### Public image registry:
 ```console
 helm install elastic-agent ../../ \
      -f ./agent-kubernetes-values.yaml \
      --set outputs.default.type=ESSecretAuthAPI \
      --set outputs.default.secretName=es-api-secret
 ```
 
+
+#### Private image registry:
+Create secret with the contents of docker auth config
+```
+kubectl create secret generic regcred --from-file=.dockerconfigjson=<your home folder here>/.docker/config.json --type=kubernetes.io/dockerconfigjson
+```
+
+Install elastic-agent
+```console
+helm install elastic-agent ../../ \
+     -f ./agent-kubernetes-values.yaml \
+     --set 'agent.imagePullSecrets[0].name=regcred' \
+     --set outputs.default.type=ESSecretAuthAPI \
+     --set outputs.default.secretName=es-api-secret
+
 ## Validate:
 
 1. The Kibana `kubernetes`-related dashboards should start showing up the respective info.