Document how to access all the LEDs together

I've left out that it's possble to access individual LEDs in the
mapping using the `KCHLED` enum for a couple of reasons:

 - I don't know if we're going to explose that enum, and
 - it's probably better that we document only a single way
   to access the LEDs, to encourage consistency between teams

Author: PeterJCLaw

programming/sr/leds/index.md

@@ -47,3 +47,15 @@ R.kch.c.rgb = (1, 1, 0)
 # Set LED B to a light blue colour
 R.kch.b.rgb = (0, 1, 1)
 ~~~~~
+
+You can also access all the LEDs together:
+
+~~~~~ python
+# Turn on the blue channel of all the LEDs
+for led in R.kch.leds.values():
+    led.blue = 1
+
+# Set all LEDs to a purple colour
+for led in R.kch.leds.values():
+    led.rgb = (1, 0, 1)
+~~~~~