update StepBar & readme

src/Shared/HandyControlDemo_Shared/ViewModel/Controls/StepBarDemoViewModel.cs

@@ -16,6 +16,18 @@ namespace HandyControlDemo.ViewModel
     {
         public StepBarDemoViewModel(DataService dataService) => DataList = dataService.GetStepBarDemoDataList();
 
+        private int _stepIndex;
+
+        public int StepIndex
+        {
+            get => _stepIndex;
+#if netle40
+            set => Set(nameof(StepIndex), ref _stepIndex, value);
+#else
+            set => Set(ref _stepIndex, value);
+#endif
+        }
+
         /// <summary>
         ///     下一步
         /// </summary>