Manual: Add experiment ready signal.

src/experiment.c

@@ -1005,6 +1005,7 @@ static void manual_experiment(void){
     tc_set_direction(&EXP_TC0_0, TC_UP);
     
     EXP_TC0_0.CNT = 0;
+    printf("R\n");  // signal ready
     
     while (1){
         if (udi_cdc_is_rx_ready()){