chore (Badge) : Add .NET foundation badge (#4550)

1. Found the .NET foundation badge at https://img.shields.io/badge/.NET%20Foundation-blueviolet.svg
2. Add to badge collection.

README.md

@@ -1,5 +1,5 @@
 # Windows Presentation Foundation (WPF)
-
+[![.NET Foundation](https://img.shields.io/badge/.NET%20Foundation-blueviolet.svg)](https://www.dotnetfoundation.org/)
 [![Build Status](https://dnceng.visualstudio.com/public/_apis/build/status/dotnet/wpf/dotnet-wpf%20CI)](https://dnceng.visualstudio.com/public/_build/latest?definitionId=270)
 [![MIT License](https://img.shields.io/badge/license-MIT-green.svg)](https://github.com/dotnet/wpf/blob/master/LICENSE.TXT)