Added additional dependencies for pydfu.

Pyboard-Firmware-Update.md

@@ -81,6 +81,10 @@ $ python pydfu.py -u <firmware.dfu>
 ```
 This will copy the given firmware file to the pyboard.  pydfu is much faster than dfu-util.
 
+Dependencies
+* libusb (MacPorts: port install libusb or HomeBrew: brew install libusb)
+* PyUSB  (pip install pyusb)
+
 ### On Windows
 
 See this PDF document: [Micro-Python-Windows-setup.pdf](http://micropython.org/resources/Micro-Python-Windows-setup.pdf)