Update AnalogRead_DigitalRead.ino

Wait for serial port to connect. Needed for native USB, on LEONARDO, MICRO, YUN, and other 32u4 based boards.
Thanks to @SalvorinFex for testing these boards.

Author: feilipu

examples/AnalogRead_DigitalRead/AnalogRead_DigitalRead.ino

@@ -14,6 +14,10 @@ void setup() {
 
   // initialize serial communication at 9600 bits per second:
   Serial.begin(9600);
+  
+  while (!Serial) {
+    ; // wait for serial port to connect. Needed for native USB, on LEONARDO, MICRO, YUN, and other 32u4 based boards.
+  }
 
   // Semaphores are useful to stop a Task proceeding, where it should be paused to wait,
   // because it is sharing a resource, such as the Serial port.