docs: update app-in-mainview-plugin description

Author: hqer927

README-zh.md

@@ -576,6 +576,8 @@ const fastboard = await createFastboard({
   fastboard.appInMainViewPlugin
   ...
 ```
+### 注意
+- 目前只有web端支持 `appInMainViewPlugin`插件, 移动端暂不支持。
 
 <h2 id="customization">自定义</h2>
 

README.md

@@ -570,6 +570,8 @@ fastboard.appInMainViewPlugin
 ...
 
 ```
+**Note:**
+- Currently, only the web end supports the 'appInMainViewPlugin' plugin, while the mobile end does not support it for the time being.
 
 ## Customization