Added Black formatting badge

Author: evaherrada

README.rst

@@ -13,6 +13,10 @@ Introduction
     :target: https://github.com/adafruit/Adafruit_CircuitPython_TrellisM4/actions/
     :alt: Build Status
 
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+    :target: https://github.com/psf/black
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 This high level library provides objects that represent Trellis M4 hardware.
 
 Dependencies