The Metal Gear Solid Facebook page has been updated with a post listing a special pre-order offer for Metal Gear Solid: Ground Zeroes available at PlayStation Store. If you purchase Ground Zeroes for PlayStation 3 on PSN anytime before the release of the game on March 18, you will receive a free download of Metal Gear Solid: Peace Walker HD.
As of right now, there is no listing for Ground Zeroes on the PlayStation Store, a situation which will likely be remedied soon, but currently renders the listed offer unavailable.
I guess that's not a bad way to get up to speed on the plot if you've not yet played Peace Walker. This offer seems to only be available on the PS3, which would make perfect sense as there's no version of Peace Walker HD available to release on PlayStation 4.