Documentation added to Deck class to indicate proper use and legal metadata.

src/main/java/forge/deck/Deck.java

@@ -13,6 +13,10 @@ import java.util.*;
 /**
  * <p>Deck class.</p>
  * 
+ * The set of MTG legal cards that become player's library when the game starts. 
+ * Any other data is not part of a deck and should be stored elsewhere.
+ * Current fields allowed for deck metadata are Name, Title, Description, Difficulty, Icon, Comment.
+ *
  * @author Forge
  * @version $Id$
  */