added gradient to using_gradient example

using_gradient/lib/main.dart

@@ -20,5 +20,13 @@ void main() {
                   "Hello World!",
                 ),
               ),
-          ))));
+              decoration: new BoxDecoration(
+                // Add Gradient
+                gradient: new LinearGradient(
+                    colors: [Colors.lightBlueAccent, Colors.blueAccent],
+                    begin: const FractionalOffset(0.0, 0.0),
+                    end: const FractionalOffset(0.6, 0.0),
+                    stops: [0.0, 0.6],
+                    tileMode: TileMode.clamp),
+              )))));
 }