Add Coordinate getter documentation

lib/src/Grid.dart

@@ -21,6 +21,12 @@ class Grid<E> extends DelegatingList<E> {
         height = h,
         super(l);
 
+  /// Return element at coordinate position
+  ///
+  /// Returns the corresponding element after checking the parameters
+  /// for the correct constraints along the width and height of the grid.
+  /// Throws [RangeError] if outside of constraints. Preferred method
+  /// to access elements via coordinates.
   E get(int x, int y) {
     int i = toIndex(x, y);
     if (i >= length || x > width - 1) throw RangeError.index(i, this);