Add FileHandle example

Author: mprse

APIs_Platform/FileHandle/README.md

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+## FileHandle example
+
+The example monitors a serial console device, and every time it outputs a character, send it to the console back and toggle LED2.
+
+1. Flash the board, and ensure the target's USB is plugged into the PC.
+2. Open serial console (select associated COM port, transmission speed: 19200 bps).
+3. Reset the target.
+4. Every time that device outputs a character, the character should appear in the console and LED2 should be toggled LED2.
+
+**Note:** You can check the associated serial port using `mbedls` command.

APIs_Platform/FileHandle/main.cpp

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+#include "mbed.h"
+
+static DigitalOut led2(LED2);
+
+// UARTSerial derives from FileHandle
+static UARTSerial device(STDIO_UART_TX, STDIO_UART_RX);
+
+int main()
+{
+    // Perform device-specific setup
+    device.set_baud(19200);
+
+    // Once set up, access through the C library
+    FILE *devin = fdopen(&device, "r");
+
+    while (1) {
+        putchar(fgetc(devin));
+        led2 = !led2;
+    }
+}