Add link to doc of notification headers

Author: laurilehmijoki

src/main/java/de/bitzeche/video/transcoding/zencoder/job/ZencoderNotification.java

@@ -73,6 +73,9 @@ public void setFormat(String format) {
 		this.format = format;
 	}
 
+    /**
+     * @param headers see https://app.zencoder.com/docs/api/encoding/notifications/notification-headers
+     */
     public void setHeaders(Map<String, String> headers) {
         this.headers = headers;
     }