Quick addition of ButtonBoard docs

Nothing special (no recipes yet); just add the stuff necessary to the
reST to pull it from source and add it to the diagrams

docs/images/composite_device_hierarchy.dot

@@ -35,4 +35,5 @@ digraph classes {
     Servo->CompositeDevice;
     AngularServo->Servo;
     Energenie->Device;
+    ButtonBoard->CompositeDevice;
 }