Add Shutdown button example to recipes, close #325

docs/recipes.rst

@@ -239,6 +239,25 @@ another to capture::
     pause()
 
 
+Shutdown button
+===============
+
+The :class:`Button` class also provides the ability to run a function when the
+button has been held for a given length of time. This example will shut down
+the Raspberry Pi when the button is held for 2 seconds::
+
+	from gpiozero import Button
+	from subprocess import check_call
+	from signal import pause
+
+	def shutdown():
+		check_call(['sudo', 'poweroff'])
+
+	shutdown_btn = Button(17, hold_time=2)
+	shutdown_btn.when_held = shutdown
+
+	pause() 
+
 LEDBoard
 ========