Update README to include Hallowing

Joshua Lowe · web-flow · commit 59e43a2be4e7 · 2018-10-23T16:05:07.000+01:00
diff --git a/README.rst b/README.rst
@@ -37,6 +37,7 @@ Designed for CircuitPython
 -  `Adafruit ItsyBitsy M0 Express <https://www.adafruit.com/product/3727>`_ (`CircuitPython Guide <https://learn.adafruit.com/introducing-itsy-bitsy-m0/circuitpython>`__)
 -  `Adafruit Trinket M0 <https://www.adafruit.com/product/3500>`__ (`CircuitPython Guide <https://learn.adafruit.com/adafruit-trinket-m0-circuitpython-arduino/circuitpython>`__)
 -  `Adafruit Metro M4 <https://www.adafruit.com/product/3382>`__ (`CircuitPython Guide <https://learn.adafruit.com/adafruit-metro-m4-express-featuring-atsamd51/overview>`__)
+-  `Adafruit Hallowing M0 Express <https://www.adafruit.com/product/3900>`__ (`CircuitPython Guide <https://learn.adafruit.com/adafruit-hallowing/circuitpython>`__)
 
 Other
 ~~~~~
