Add comment regarding flushing producer

include/cppkafka/producer.h

@@ -74,6 +74,9 @@ class Message;
  * // Write using a key on a fixed partition (42)
  * producer.produce(MessageBuilder("some_topic").partition(42).key(key).payload(payload));
  *
+ * // Flush the produced messages
+ * producer.flush();
+ * 
  * \endcode
  */
 class CPPKAFKA_API Producer : public KafkaHandleBase {