add serial example

[challenge-bot] / serial / README

(how to use this
 (connect your robot to your computer via usb cable)
 (open arduino ide)
 (in arduino ide, open serial.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")
 (hit the reset button on the arduino)
 (you should see "hello computer!" each time you hit the reset button)
 (yaaay!))