uart_setup: add example on current uart use.

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>

api/mraa/uart.h

@@ -31,6 +31,8 @@
  * UART is the Universal asynchronous receiver/transmitter interface to
  * libmraa. It allows the exposure of UART pins on supported boards.
  * With functionality to expand at a later date.
+ *
+ * @snippet uart_setup.c Interesting
  */
 
 #ifdef __cplusplus