Update sourcevalues docs as suggested in lurch's review

docs/examples/source_value_processing.py

@@ -0,0 +1,13 @@
+from gpiozero import Button, LED
+from signal import pause
+
+def opposite(values):
+    for value in values:
+        yield not value
+
+led = LED(4)
+btn = Button(17)
+
+led.source = opposite(btn.values)
+
+pause()