add framebuf dep

Author: ladyada

README.rst

@@ -26,6 +26,7 @@ This driver depends on:
 
 * `Adafruit CircuitPython <https://github.com/adafruit/circuitpython>`_
 * `Bus Device <https://github.com/adafruit/Adafruit_CircuitPython_BusDevice>`_
+* `Adafruit framebuf <https://github.com/adafruit/Adafruit_CircuitPython_framebuf>`_
 
 Please ensure all dependencies are available on the CircuitPython filesystem.
 This is easily achieved by downloading