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

OpenRA.Mods.RA/Crate.cs

@@ -56,6 +56,8 @@ namespace OpenRA.Mods.RA
 			this.Info = info;
 		}
 
+		public void WarnCrush(Actor crusher) {}
+
 		public void OnCrush(Actor crusher)
 		{
 			var shares = self.TraitsImplementing<CrateAction>().Select(