chore: add per-user prop example to alerting guide (#777)

content/guides/alerting.mdx

@@ -133,9 +133,20 @@ Subscriptions can also hold their [own unique properties](/concepts/subscription
 ```javascript title="Subscribe a user to an alert Object"
 knock.objects.addSubscriptions("alerts", alertId, {
   recipients: ["user_79bc96a9", "user_JG9NGAJQ", "user_391d92cd"],
+  properties: {
+    channels: ["email", "in_app"],
+    events: ["maintenance", "compliance"],
+    batchWindow: {
+      frequency: "weekly",
+      days: ["fri"],
+      hours: 17,
+    },
+  },
 });
 ```
 
+Once you've stored these properties on a subscription, you can access them inside the workflow run on `recipient.subscription.channel` or `recipient.subscription.events` for use in either templates or step conditions. This allows you to create a highly configurable alerting system that can be customized on a per-user basis.
+
 ### Triggering an alert
 
 To trigger an alert workflow, your application would only need the `id` of the alert you want to trigger. From there, you would fetch the `alert` Object from Knock (we’re working on some ways to more easily reference objects inside of workflow runs).