Added basic driver example use code to the library documentation.

Made the USARTStream global public and documented in the SerialStream module, allowing for the serial USART stream to be accessed via its handle rather than via the implicit stdout and stdin streams.

LUFA/Drivers/Peripheral/Serial.h

@@ -45,6 +45,18 @@
  *  Hardware serial USART driver. This module provides an easy to use driver for
  *  the setup of and transfer of data over the AVR's USART port.
  *
+ *  <b>Example Usage:</b>
+ *  \code
+ *      // Initialise the serial USART driver before first use, with 9600 baud (and no double-speed mode)
+ *      Serial_Init(9600, false);
+ *
+ *      // Send a string through the USART
+ *      Serial_TxString("Test String\r\n");
+ *
+ *      // Receive a byte through the USART
+ *      uint8_t DataByte = Serial_RxByte();
+ *  \endcode
+ *
  *  @{
  */