add messageloaded webhook event details

closes postal/postal#629

Author: adamcooke

content/4.developer/4.webhooks.md

@@ -98,6 +98,29 @@ If you have click tracking enabled, the `MessageLinkClicked` event will tell you
 }
 ```
 
+## Message Loaded/Opened Event
+
+If you have open tracking enabled, the `MessageLoaded` event will tell you that a user has opened your e-mail (or, at least, have viewed the tracking pixel embedded within it.)
+
+```javascript
+{
+  "ip_address":"185.22.208.2",
+  "user_agent":"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.98 Safari/537.36",
+  "message":{
+    "id":12345,
+    "token":"abcdef123",
+    "direction":"outgoing",
+    "message_id":"[email protected]",
+    "to":"[email protected]",
+    "from":"[email protected]",
+    "subject":"Welcome to AwesomeApp",
+    "timestamp":1477945177.12994,
+    "spam_status":"NotSpam",
+    "tag":"welcome"
+  }
+}
+```
+
 ## DNS Error Event
 
 Postal regularly monitors domains it knows about to ensure that your SPF/DKIM/MX records are correct. If you'd like to be notified when the checks fail, you can subscribe to the `DomainDNSError` event.