added ref property

Author: 256dpi

README.md

@@ -139,6 +139,7 @@ void onMessageAdvanced(MQTTClientCallbackAdvanced);
 ```
 
 - The set callback is mostly called during a call to `loop()` but may also be called during a call to `subscribe()`, `unsubscribe()` or `publish() // QoS > 0` if messages have been received before receiving the required acknowledgement. Therefore, it is strongly recommended to not call `subscribe()`, `unsubscribe()` or `publish() // QoS > 0` directly in the callback.
+- In case you need a reference to an object that manages the client, use the `void * ref` property on the client to store a pointer, and access it directly from the advanced callback..
 
 Set more advanced options:
 

src/MQTTClient.h

@@ -57,6 +57,8 @@ class MQTTClient {
   lwmqtt_err_t _lastError = (lwmqtt_err_t)0;
 
  public:
+  void * ref;
+
   explicit MQTTClient(int bufSize = 128);
 
   ~MQTTClient();