Add comment on how to set the used UART port (#39)

Author: triveria

microros_extensions/microros_transports.h

@@ -43,6 +43,7 @@ typedef struct {
 
 #if defined(MICRO_ROS_TRANSPORT_SERIAL)
     #define MICRO_ROS_FRAMING_REQUIRED true
+    // Set the UART used for communication. e.g. {.fd = 0} leads to UART_0 being used.
     static zephyr_transport_params_t default_params = {.fd = 0};
 #elif defined(MICRO_ROS_TRANSPORT_UDP)
     #define MICRO_ROS_FRAMING_REQUIRED false
@@ -61,4 +62,4 @@ size_t zephyr_transport_read(struct uxrCustomTransport* transport, uint8_t* buf,
 }
 #endif
 
-#endif //_MICROROS_CLIENT_ZEPHYR_SERIAL_TRANSPORT_H_
+#endif //_MICROROS_CLIENT_ZEPHYR_SERIAL_TRANSPORT_H_