Add source/values docs page, close #448

docs/examples/custom_generator.py

@@ -0,0 +1,12 @@
+from gpiozero import LED
+from random import randint
+from signal import pause
+
+def rand():
+    while True:
+        yield randint(0, 1)
+
+led = LED(17)
+led.source = rand()
+
+pause()