Add .NET Book Zero, by Charles Petzold (#5656)

books/free-programming-books-langs.md

@@ -458,6 +458,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Introduction to Rx](http://www.introtorx.com)
 * [Learn C# in Y Minutes](https://learnxinyminutes.com/docs/csharp/)
 * [Mastering Xamarin UI Development, Second Edition](https://www.packtpub.com/free-ebooks/mastering-xamarin-ui-development-second-edition) - Steven F. Daniel (Packt account *required*)
+* [.NET Book Zero](http://www.charlespetzold.com/dotnet) - Charles Petzold (PDF, XPS)
 * [Object Oriented Programming using C#](http://bookboon.com/en/object-oriented-programming-using-c-sharp-ebook) - Simon Kendal, Bookboon. (email address *requested*, not required)
 * [Threading in C#](http://www.albahari.com/threading/)
 * [Xamarin.Forms for macOS Succinctly](https://www.syncfusion.com/ebooks/xamarin_forms_for_mac_os_succinctly) - Alessandro Del Sole