Moisture Barrier Under Shed

I'm purchasing a pre-fabricated vinyl-sided wood frame storage shed that is constructed on 4 x 4 P.T. skids. I plan on putting down a 3-4 inch base of 3/4 inch limestone on which to set the building. My question regards installing a moisture barrier under the shed. Is it a good idea? If so, should the barrier be placed on top of the rock as is commonly done in a crawl space for a raised foundation house? Or should it be place below the rock but on top of the soil? Thanks in advance for your advice.

Paul