Add mg_mqtt_pubex

docs/README.md

@@ -982,6 +982,15 @@ void mg_mqtt_pub(struct mg_connection *, struct mg_str *topic,
 
 Publish message `data` to the topic `topic`.
 
+### mg\_mqtt\_pubex()
+
+```c
+void mg_mqtt_pubex(struct mg_connection *, struct mg_str *topic,
+                   struct mg_str *data, int qos, bool retain);
+```
+
+Like `mg_mqtt_pub()` but also allows to set QoS and retain flag.
+
 
 ### mg\_mqtt\_sub()