add Note about XAML designers w/ Xamarin.Forms

Author: TerryGLee

docs/xaml-tools/creating-a-ui-by-using-xaml-designer-in-visual-studio.md

@@ -1,6 +1,6 @@
 ---
 title: XAML Designer overview
-ms.date: 07/31/2019
+ms.date: 03/03/2020
 ms.topic: conceptual
 f1_keywords:
 - VS.XamlDesigner
@@ -19,6 +19,9 @@ The XAML Designer in Visual Studio and Blend for Visual Studio provides a visual
 
 For advanced users, you can even [customize the XAML Designer](https://github.com/microsoft/xaml-designer-extensibility/blob/master/documents/xaml-designer-extensibility-migration.md).
 
+> [!NOTE]
+> Xamarin.Forms does not support a XAML designer. To view your Xamarin.Forms XAML UIs and edit them while the app is running, use XAML Hot Reload for Xamarin.Forms. For more information, see the [XAML Hot Reload for Xamarin.Forms (Preview)](/xamarin/xamarin-forms/xaml/hot-reload/) page."
+
 ## XAML Designer workspace
 
 The workspace in XAML Designer consists of several visual interface elements. These include the *artboard* (which is the visual design surface), XAML editor, Document Outline window (Objects and Timeline window in Blend for Visual Studio), and Properties window. To open the XAML Designer, right-click a XAML file in **Solution Explorer** and choose **View Designer**.