Update docs: add PWMLED and include value, values and source properties

docs/recipes.md

@@ -494,3 +494,20 @@ while True:
     led.green = green_pot.value
     led.blue = blue_pot.value
 ```
+
+Alternatively, the following example is identical and uses the `source` property
+rather than a `while` loop:
+
+```python
+from gpiozero import RGBLED, MCP3008
+from signal import pause
+
+led = RGBLED(red=2, green=3, blue=4)
+red_pot = MCP3008(channel=0)
+green_pot = MCP3008(channel=1)
+blue_pot = MCP3008(channel=2)
+
+led.red.source = red_pot.values
+led.green.source = green_pot.values
+led.blue.source = blue_pot.values
+```