adds comment on pawn placement

src/grid.rs

@@ -46,6 +46,13 @@ impl AsRef<gtk::EventBox> for Cell {
     }
 }
 
+
+/// The cell widget is either empty or has a pawn
+/// Pawn can be moved around cells using drag-n-drop. However, due to my inability
+/// to dynamically change the behavior with set/unset methods, all cells are always
+/// drag and drop at once.
+/// It is usable though, just weird. When you grab an empty cell you'll make a button appear.
+/// Know that if you drop a cell on itself, it will empty itself !
 #[derive(Debug, Clone)]
 struct CellWidget {
     eventbox: gtk::EventBox,