Update the nuget readme with instructions of how to do app level overrides of Winui2 resources. (#4279)

* Update the nuget readme with instuctions of how to do app level overrides of Winui2 resources.

* Fix some mistakes

* Fix the root of the overrides example

* Update build/NuSpecs/readme.txt

Co-authored-by: Marcel Wagner <marcel.alex.wagner@outlook.com>

Co-authored-by: Marcel Wagner <marcel.alex.wagner@outlook.com>

Author: StephenLPeters

build/NuSpecs/readme.txt

@@ -8,16 +8,20 @@ Don't forget to set XamlControlsResources as your Application resources in App.x
         </Application.Resources>
     </Application>
 
-or if you have other resources then add XamlControlsResources at the top as a merged dictionary:
+If you have other resources, then we recommend you add those to the XamlControlsResources' MergedDictionaries.
+This works with the platform's resource system to allow overrides of the XamlControlsResources resources.
 
+<Application
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:controls="using:Microsoft.UI.Xaml.Controls">
     <Application.Resources>
-        <ResourceDictionary>
-            <ResourceDictionary.MergedDictionaries>
-                <XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls" />
-                <!-- Other merged dictionaries here -->
-            </ResourceDictionary.MergedDictionaries>
-            <!-- Other app resources here -->
-        </ResourceDictionary>
+        <controls:XamlControlsResources>
+            <controls:XamlControlsResources.MergedDictionaries>
+                <!-- Other app resources here -->
+            </controls:XamlControlsResources.MergedDictionaries>
+        </controls:XamlControlsResources>
     </Application.Resources>
+</Application>
 
 See http://aka.ms/winui for more information.