update the doc

Author: jinfahua

README.md

@@ -128,7 +128,7 @@ For **Share conn in thread**, please refer to the Pub sampler.
 
 -  **QoS level**: The available QoS value, 0 is AT_MOST_ONCE, 1 is AT_LEAST_ONCE and 2 is EXACTLY_ONCE.
 
--  **Topic name**: The topic name that subscriber will subscribe to.
+-  **Topic name**: The topic name that subscriber will subscribe to. If want to subscribe multiple topics, please use comma to separate the different topics.
 
 -  **Payload includes timestamp**: If the checkbox is enabled, then it means the payload includes timestamp. It can be used to calcuate the message latency time.