Add a WarnCrush method to ICrushable which is called when a crusher begins to enter the cell.

OpenRA.Game/Traits/TraitsInterfaces.cs

@@ -142,6 +142,7 @@ namespace OpenRA.Traits
     public interface ICrushable
     {
         void OnCrush(Actor crusher);
+		void WarnCrush(Actor crusher);
 		bool CrushableBy(string[] crushClasses, Player owner);
     }