Add wpf and xamarin topics as related

Author: worldbeater

topics/avalonia/index.md

@@ -4,6 +4,7 @@ created_by: Steven Kirk
 display_name: Avalonia
 github_url: https://github.com/avaloniaui/avalonia
 logo: avalonia.png
+related: wpf, xamarin
 released: 2016
 short_description: A cross-platform XAML UI framework for .NET Core, .NET Framework and Mono.
 topic: avalonia