update readme

README-ZH.md

 
 如果想在runtime选择是否debug，请修改文件```com.unity.uiwidgets/com.unity.uiwidgets/Runtime/foundation/debug.cs```中的```static bool debugEnableAtRuntimeInternal```
 
+## 使用Window Scope保护外部调用
+如果您在调试时遇到 `AssertionError: Window.instance is null` 或者在调用 `Window.instance` 时得到空指针, 那么您需要
+使用以下方式来保护您的调用，使之可以在正确的Isolate上执行回调逻辑：
+```csharp
+using(Isolate.getScope(the isolate of your App)) {
+    // code dealing with UIWidgets,
+    // e.g. setState(() => {....})
+}
+```
+
+通常，在您使用外部事件，例如来自UIWidgets之外的输入事件、网络传输回调事件时需要做这样的处理。具体的您可以参考我们的 HttpRequestSample 这个样例中的写法。需要注意的是，一般情况下您在UIWidgets框架的内部调用 (如 `Widget.build, State.initState...`)中不需要额外采取上述保护措施。
+
 ## 学习
 
 #### 教程

README.md

 
 If you want to change different mode in runtime, please modify the file “com.unity.uiwidgets/com.unity.uiwidgets/Runtime/foundation/debug.cs” by making “static bool debugEnableAtRuntimeInternal” to true or false. Note that the value is set to false by default.
 
+## Using Window Scope
+If you see the error `AssertionError: Window.instance is null` or null pointer error of `Window.instance`,
+it means the code is not running in the window scope. In this case, you can enclose your code
+with window scope as below:
+```csharp
+using(Isolate.getScope(the isolate of your App)) {
+    // code dealing with UIWidgets,
+    // e.g. setState(() => {....})
+}
+```
+
+This is needed if the code is in methods
+not invoked by UIWidgets. For example, if the code is in `completed` callback of `UnityWebRequest`,
+you need to enclose them with window scope.
+Please see our HttpRequestSample for detail.
+For callback/event handler methods from UIWidgets (e.g `Widget.build, State.initState...`), you don't need do
+it yourself, since the framework ensure it's in window scope.
+
 ## Learn
 
 #### Samples