Fan extender for RAMPS 1.4

A small circuit to add a second fan to your RAMPS 1.4 controller.

For instance :

* the first fan is wired to D9 and cools down the print
* the second fan is wired to the fan extender and cools down the printing head

The schematic is the same as on RAMPS board (for D8 to D10). A LED indicates when the fan is running.