Not sure about wiring a power switch to the GPIO pins. I think the answer to that question would be answered better on an Intel Edison forum. On my own Reach modules, I make use of the switch on my power supply when I can. Emlid has previously stated that power disconnect is an approved method of powering off the Reach module, and that they have taken steps to counteract loss of data.
I did a little searching and I didn’t find any code written for the Edison to trigger a shutdown from GPIO, so I think you’d have to write it yourself. What I did find is code that will trigger a GPIO output when Edison is shutting down (to tell a power supply to turn itself off). That is the opposite of what you are looking for though.
a pull up resistor tied to Vcc (3.3V) and + side of switch
a connection between a GPIO pin and the + side of switch
a connection to GND (0V) and the - side of switch
And then there is the concern of debouncing the switch contacts. If all the switch does is signal a shutdown command, then it should not matter. But if you want a switch to do any other function, then your switch should have both NO and NC contacts, plus it should be wired to an IC with logic gates before being connected to the GPIO pin. This advice could be from outdated knowledge though. Maybe debouncing can be taken care of in another way these days.