Add confirm-rollout hook to docs

docs/gitbook/how-it-works.md

@@ -655,6 +655,8 @@ The canary analysis can be extended with webhooks. Flagger will call each webhoo
 determine from the response status code (HTTP 2xx) if the canary is failing or not.
 
 There are three types of hooks:
+* Confirm-rollout hooks are executed before scaling up the canary deployment and ca be used for manual approval.
+The rollout is paused until the hook returns a successful HTTP status code.
 * Pre-rollout hooks are executed before routing traffic to canary. 
 The canary advancement is paused if a pre-rollout hook fails and if the number of failures reach the 
 threshold the canary will be rollback.
@@ -668,6 +670,9 @@ Spec:
 ```yaml
   canaryAnalysis:
     webhooks:
+      - name: "start gate"
+        type: confirm-rollout
+        url: http://flagger-loadtester.test/gate/approve
       - name: "smoke test"
         type: pre-rollout
         url: http://flagger-helmtester.kube-system/