Add requirement of Adafruit_BusIO

Author: samyk

README.md

@@ -13,6 +13,8 @@ products from Adafruit!
 
 Check out the links above for our tutorials and wiring diagrams
 
+Requires the https://github.com/adafruit/Adafruit_BusIO library for I2C abstraction
+
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 please support Adafruit and open-source hardware by purchasing
 products from Adafruit!