What is the reference pin on an instrumentation amplifier used for?
The reference pin is exactly for level shifting the output. Note that it is best
to use a low impedance source for this.An op amp as a unity gain buffer or other configuration should ideally drive this pin.

For decoupling take a look at the typical application circuit in the datasheet. It recommends a 0.1uF capacitor. Perhaps an extra 10uF capacitor would also be good.