Update Create-UI-Controls.md

Author: karann-msft

docs/guides/Create-UI-Controls.md

@@ -82,7 +82,9 @@ In the following example, the control implemented in `ManagedPackage.winmd` will
 
 ## Add custom icons to your controls
 
-To display a custom icon in the toolbox/assets pane, add an image to your project or the corresponding `design.dll` project with the name “Namespace.ControlName.extension” and set the build action to “Embedded Resource”. Supported formats are `.png`, `.jpg`, `.jpeg`, `.gif`, and `.bmp`. The recommended image size is 64 pixels by 64 pixels.
+To display a custom icon in the toolbox/assets pane, add an image to your project or the corresponding `design.dll` project with the name “Namespace.ControlName.extension” and set the build action to “Embedded Resource”. You must also ensure that the associated `AssemblyInfo.cs` specifies the ProvideMetadata attribute - `[assembly: ProvideMetadata(typeof(RegisterMetadata))]`. See this [sample](https://github.com/NuGet/Samples/blob/master/ExtensionSDKasNuGetPackage/NativePackage.Design/Properties/AssemblyInfo.cs#L20).
+
+Supported formats are `.png`, `.jpg`, `.jpeg`, `.gif`, and `.bmp`. The recommended image size is 64 pixels by 64 pixels.
 
 In the example below, the project contains an image file named “ManagedPackage.MyCustomControl.png”.