added readme

Upgrading/CustomRenderer/MultiProject/README.md

@@ -0,0 +1,17 @@
+---
+name: Using Custom Renderers in .NET MAUI with multiple projects
+description: "Sample for using custom renderers in .NET MAUI."
+page_type: sample
+languages:
+- csharp
+- xaml
+products:
+- dotnet-maui
+- dotnet-core
+urlFragment: custom-renderers
+---
+
+# Custom Renderers with Multiple Projects
+
+When upgrading from Xamarin.Forms to .NET MAUI, custom renderers can be left in the platform projects. To register them, add a `MauiProgram.cs` file to each project and apply the platform specific code there. This sample demonstrates this approach.
+