Add docs for automation MQTT trigger (#1593)

source/getting-started/automation-templating.markdown

@@ -68,6 +68,7 @@ The following tables show the available trigger data per platform.
 | `trigger.platform` | Hardcoded: `mqtt`.
 | `trigger.topic` | Topic that received payload.
 | `trigger.payload` | Payload.
+| `trigger.payload_json` | Dictonary of the JSON parsed payload.
 | `trigger.qos` | QOS of payload.
 
 ### {% linkable_title numeric_state %}