Additional Rules from the Rulebook*Edit
If you do not have 5 cards in your deck, reveal as many as you can and shuffle your discard pile to reveal the rest. The player to your left then chooses one of the revealed cards for your to discard and then you keep the rest. If you do not have enough cards left to reveal 5 cards, even after shuffling, reveal as many as you can. The opponent to your left still discards one card before you draw the rest.
(* a sheet containing the additional explanation above was included in the Amazon Big Box exclusive set containing Dominion, Alchemy and Prosperity.)