Hi there,
I've just built myself pretty much the same thing as what you have, kk2.1.5, hobby king 6ch transmitter. Each channel of the receiver has 3 pins, 2 of these are 5v 0v, which it will then power itself from the flight controller. The input marked BAT was used to bind the transmitter and receiver.
Was also wondering what this board does when out of range or on low battery. I haven't actually had the chance to fly mine yet, still waiting on the usb cable coming so I can reverse 2 of the channels :/