Add customcontrol sample to readme (#201)

Author: angelazhangmsft

README.md

@@ -27,6 +27,9 @@ This repository hosts samples for the [Windows App SDK](https://github.com/micro
 #### Notifications
 - [Push Notifications](Samples/Notifications/Push): This is a sample app that showcases Push Notifications.
 
+#### Runtime Components
+- [Custom Controls](Samples/CustomControls): This sample shows how to author a Windows Runtime Component in C# with custom WinUI controls.
+
 #### User Interface and Input
 - [Windowing](Samples/Windowing): This sample demonstrates how to manage app windows using the Windowing APIs.
 - [Windows Input and Composition Gallery](https://github.com/microsoft/WindowsCompositionSamples): This collection of samples showcases Microsoft.UI.Composition and Microsoft.UI.Input APIs.