add dep to readme dox

Author: ladyada

README.md

@@ -10,6 +10,8 @@ All text above must be included in any redistribution.
 
 Recent Arduino IDE releases include the Library Manager for easy installation. Otherwise, to download, click the DOWNLOAD ZIP button, uncompress and rename the uncompressed folder Adafruit_GFX. Confirm that the Adafruit_GFX folder contains Adafruit_GFX.cpp and Adafruit_GFX.h. Place the Adafruit_GFX library folder your ArduinoSketchFolder/Libraries/ folder. You may need to create the Libraries subfolder if its your first library. Restart the IDE.
 
+**You will also need to install the latest Adafruit BusIO library.** Search for "Adafruit BusIO" in the library manager, or install by hand from https://github.com/adafruit/Adafruit_BusIO
+
 # Useful Resources
 
 - Image2Code: This is a handy Java GUI utility to convert a BMP file into the array code necessary to display the image with the drawBitmap function. Check out the code at ehubin's GitHub repository: https://github.com/ehubin/Adafruit-GFX-Library/tree/master/Img2Code