Updated Design: Dynamic MQTT node (markdown)

Design:-Dynamic-MQTT-node.md

@@ -27,3 +27,86 @@ Some observations from this design:
 
  - the control node is controlling the `mqtt-broker` node. Other regular nodes can also use that `mqtt-broker` node so will also be effected by changes.
  - the `mqtt-broker` config node will have a new option to not auto-connect. This would be used where the decision to connect is left to the application's use of the control node.
+
+---
+
+### `msg.mqttControl`
+
+This message property is used to control the `mqtt-control` node. It's content determines the action the control node will take.
+
+#### Connect
+
+The connect action will connect the mqtt-control node if it is not currently connected.
+
+If the broker is already connected, the action will be ignored. (**Q**: or should it cause a disconnect and reconnect?)
+
+The `broker` object can contain any property to override the mqtt-broker node.
+
+~~~
+msg.mqttControl = {
+   "action": "connect",
+   "broker": {
+      "url": "mqtt://localhost:1883",
+      "clientid": "my-client-id",
+      "keepalive": 60,
+      "clean": true
+      "username": "",
+      "password": "",
+      "birth": {
+         "topic":"",
+         "qos": "",
+         "retain": false,
+         "payload": ""
+      },
+      "close": {
+         "topic":"",
+         "qos": "",
+         "retain": false,
+         "payload": ""
+      },
+      "will": {
+         "topic":"",
+         "qos": "",
+         "retain": false,
+         "payload": ""
+      }
+   }
+}
+~~~
+
+#### Disconnect
+
+~~~
+msg.mqttControl = {
+   "action": "disconnect"
+}
+~~~
+
+#### Subscribe
+
+~~~
+msg.mqttControl = {
+   "action": "subscribe",
+   "topic": "a/b/c"
+}
+~~~
+
+#### Unsubscribe
+
+~~~
+msg.mqttControl = {
+   "action": "unsubscribe",
+   "topic": "a/b/c"
+}
+~~~
+
+#### Publish
+
+The publish action will use the same standard properties as the existing `mqtt out` node. But the `msg.mqttControl` property *must* be present to tell the control node to publish.
+~~~
+msg.mqttControl = {
+   "action": "publish"
+}
+~~~
+
+