Merge pull request microsoft#4401 from microsoft/fix-4400

add note about migrating the Landscape Mountains project

docs/unreal_custenv.md

@@ -16,6 +16,10 @@ There is no `Epic Games Launcher` for Linux which means that if you need to crea
 1. Open `LandscapeMountains.uproject`, it should launch the Unreal Editor.
 
     ![unreal editor](images/unreal_editor.png)
+
+    !!!note
+
+        The Landscape Mountains project is supported up to Unreal Engine version 4.24. If you do not have 4.24 installed, you should see a dialog titled `Select Unreal Engine Version` with a dropdown to select from installed versions. Select 4.25 or greater to migrate the project to a supported engine version. If you have 4.24 installed, you can manually migrate the project by navigating to the corresponding .uproject file in Windows Explorer, right-clicking it, and selecting the `Switch Unreal Engine version...` option. 
 
 1. From the `File menu` select `New C++ class`, leave default `None` on the type of class, click `Next`, leave default name `MyClass`, and click `Create Class`. We need to do this because Unreal requires at least one source file in project. It should trigger compile and open up Visual Studio solution `LandscapeMountains.sln`.