Add CircuitPython Essentials Learn guide link to pwmio

Author: tekktrik

shared-bindings/pwmio/__init__.c

@@ -57,7 +57,12 @@
 //| :py:data:`~pwmio.PWMOut.duty_cycle`, and then sleep 0.1 seconds.
 //| CircuitPython will automatically turn off the PWM when it resets all
 //| hardware after program completion. Use ``deinit()`` or a ``with`` statement
-//| to do it yourself."""
+//| to do it yourself.
+//|
+//| For the essentials of `pwmio`, see the CircuitPython Essentials
+//| Learn guide:
+//| https://learn.adafruit.com/circuitpython-essentials/circuitpython-pwm
+//| """
 //|
 
 STATIC const mp_rom_map_elem_t pwmio_module_globals_table[] = {