Explanation of SLP jumper

[gcl8a]

I don't have the new board. Please add a description of where to put the jumper to disable the motors. And a picture of the jumper in place.

[madhephaestus]

added image https://github.com/WPIRoboticsEngineering/ZenRobotBuildKit/blob/master/photos/shim/0025.jpg to instructions with the jumper on