add docs for kubectl in loadtester

Signed-off-by: Sanskar Jaiswal <jaiswalsanskar078@gmail.com>

docs/gitbook/usage/webhooks.md

@@ -387,6 +387,22 @@ This can be done via mounting a Kubernetes secret in the tester's Deployment.
 to see if the process has finished (Default is 5s). `pollTimeout` represents the time in seconds
 the web-hook will try to call Concord before timing out (Default is 30s).
 
+If you need to start a Pod/Job to run tests, you can do so using `kubectl`.
+
+```yaml
+ analysis:
+ webhooks:
+ - name: "smoke test"
+ type: pre-rollout
+ url: http://flagger-kubectltester.kube-system/
+ timeout: 3m
+ metadata:
+ type: "kubectl"
+ cmd: "run test --image=alpine --overrides='{ "spec": { "serviceAccount": "default:default" } }'"
+```
+
+Note that you need to setup RBAC for the load tester service account in order to run `kubectl` and `helm` commands.
+
 ## Manual Gating
 
 For manual approval of a canary deployment you can use the `confirm-rollout` and `confirm-promotion` webhooks.