add pushbutton example

[challenge-bot] / pushbutton / README

(how to use this
 (connect your robot to your computer via usb cable)
 (open arduino ide)
 (in arduino ide, open pushbutton.ino)
 (hit the upload button)
 (hit control+shift+m,
  or go to the menu item tools, and down to serial monitor)
 (in the bottom right, select "9600 baud")
 (connect the breadboard and pushbutton as shown in "pushed.jpg")
 (hit the upload button in the arduino ide)
 (press the ))