Add information on canio

shared-bindings/canio/__init__.c

@@ -54,6 +54,9 @@
 //| Other implementations of the CAN device may exist (for instance, attached
 //| via an SPI bus).  If so their constructor arguments may differ, but
 //| otherwise we encourage implementors to follow the API that the core uses.
+//|
+//| For more information on working with this module, refer to
+//| `this Learn Guide on using it <https://learn.adafruit.com/using-canio-circuitpython>`_.
 //| """
 //|