Add a hook example for sending a Slack message (#2414)

docs/user-guide/resource_hooks.md

@@ -77,3 +77,55 @@ field in the spec, which let their respective controllers delete the Job/Workflo
 spec:
   ttlSecondsAfterFinished: 600
 ```
+
+## Using A Hook To Send A Slack Message
+
+The following example uses the Slack API to send a a Slack message when sync completes or fails: 
+
+```yaml
+apiVersion: batch/v1
+kind: Job
+metadata:
+  generateName: app-slack-notification-
+  annotations:
+    argocd.argoproj.io/hook: PostSync
+    argocd.argoproj.io/hook-delete-policy: HookSucceeded
+spec:
+  template:
+    spec:
+      containers:
+      - name: slack-notification
+        image: appropriate/curl
+        command:
+          - "curl"
+          - "-X"
+          - "POST"
+          - "--data-urlencode"
+          - "payload={\"channel\": \"#somechannel\", \"username\": \"hello\", \"text\": \"App Sync succeeded\", \"icon_emoji\": \":ghost:\"}"
+          - "https://hooks.slack.com/services/..."
+      restartPolicy: Never
+  backoffLimit: 2
+---
+apiVersion: batch/v1
+kind: Job
+metadata:
+  generateName: app-slack-notification-fail-
+  annotations:
+    argocd.argoproj.io/hook: SyncFail
+    argocd.argoproj.io/hook-delete-policy: HookSucceeded
+spec:
+  template:
+    spec:
+      containers:
+      - name: slack-notification
+        image: appropriate/curl
+        command: 
+          - "curl"
+          - "-X"
+          - "POST"
+          - "--data-urlencode"
+          - "payload={\"channel\": \"#somechannel\", \"username\": \"hello\", \"text\": \"App Sync failed\", \"icon_emoji\": \":ghost:\"}"
+          - "https://hooks.slack.com/services/..."
+      restartPolicy: Never
+  backoffLimit: 2
+  ```