Add link to busio from board module

Author: tekktrik

shared-bindings/board/__init__.c

@@ -43,6 +43,10 @@
 //| Common container for board base pin names. These will vary from board to
 //| board so don't expect portability when using this module.
 //|
+//| The most common use of this module is to use serial communciation buses with
+//| the default pins and settings.  For more information about serial communcication
+//| in CircuitPython, see the :mod:`busio`.
+//|
 //| .. warning:: The board module varies by board. The APIs documented here may or may not be
 //|              available on a specific board."""