Add USBCDC example

Author: mprse

APIs_USB/USBCDC/README.md

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+## USBCDC example
+
+The example sends string in the loop using an emulated serial port over USB.
+
+1. Flash the board, and ensure the target's auxiliary USB is plugged into the PC.
+2. Open serial console (select associated COM port, transmission speed: 9600 bps).
+3. Reset the target.
+4. "Hello world" string should appear in the loop on the serial console in the 1-second delays.
+
+**Note:** On Windows you can check the associated COM port in the Device Manager (USB Serial Device).
+**Note:** If you want printf functionality, please see the USBSerial class.

APIs_USB/USBCDC/main.cpp

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+#include "mbed.h"
+#include "USBSerial.h"
+
+//Virtual serial port over USB
+USBSerial serial;
+
+int main(void) {
+    while(1)
+    {
+        serial.printf("I am a virtual serial port\r\n");
+        wait(1.0);
+    }
+}