Add code snippet for UnbufferedSerial API documentation

Author: hugueskamba

APIs_Drivers/UnbufferedSerial/main.cpp

@@ -0,0 +1,35 @@
+#include "mbed.h"
+
+
+// Create a DigitalOutput object to blink an LED whenever data is received
+static DigitalOut led_serial_read(LED1);
+
+// Create a new UnbufferedSerial object with a default baud rate.
+static UnbufferedSerial serial_port(USBTX, USBRX);
+
+void on_rxd_interrupt()
+{
+    char c;
+
+    // Toggle an LED.
+    led_serial_read = !led_serial_read;
+
+    // Read the data to clear the receive interrupt.
+    if (serial_port.read(&c, 1)) {
+        // Echo the input back to the terminal.
+        serial_port.write(&c, 1);
+    }
+}
+
+int main(void) {
+    // Set desired properties (9600-8-N-1).
+    serial_port.baud(9600);
+    serial_port.format(
+        /* bits */ 8,
+        /* parity */ SerialBase::None,
+        /* stop bit */ 1
+    );
+
+    // Register a callback to process a Rx (receive) interrupt.
+    serial_port.attach(&on_rxd_interrupt, SerialBase::RxIrq);
+}