update the free-programming-books-langs.md with link to contributing guide (#6261)

books/free-programming-books-langs.md

@@ -473,6 +473,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Learn C# in Y Minutes](https://learnxinyminutes.com/docs/csharp/)
 * [Mastering Xamarin UI Development, Second Edition](https://www.packtpub.com/free-ebook/mastering-xamarin-ui-development-second-edition/9781788995511) - Steven F. Daniel (Packt account *required*)
 * [.NET Book Zero](http://www.charlespetzold.com/dotnet) - Charles Petzold (PDF, XPS)
+* [.NET Microservices: Architecture for Containerized .NET Applications](https://dotnet.microsoft.com/download/e-book/microservices-architecture/pdf) - Cesar de la Torre, Bill Wagner, Mike Rousos (PDF)
 * [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
 * [Xamarin.Forms Succinctly](https://www.syncfusion.com/ebooks/xamarin-forms-succinctly) - Alessandro Del Sole