Added "High level async programming with Rx" (#2471)

* "High level async programming with Rx" added

* Updated .NET Rx book with author

free-programming-books.md

@@ -888,6 +888,7 @@
 * [Data Structures and Algorithms with Object-Oriented Design Patterns in C#](http://www.brpreiss.com/books/opus6/)
 * [Dissecting a C# Application](https://damieng.com/blog/2007/11/08/dissecting-a-c-application-inside-sharpdevelop) - Christian Holm, Bernhard Spuida, Mike Kruger
 * [Fundamentals of Computer Programming with C#](http://www.introprogramming.info/english-intro-csharp-book/read-online/) - Svetlin Nakov
+* [High level asynchronous programming with Reactive Extensions](https://github.com/petroemil/Rx.Book) - Emil Petro
 * [Introduction to Rx](http://www.introtorx.com)
 * [Threading in C#](http://www.albahari.com/threading/)
 * [Xamarin Cross-Platform Development Cookbook](https://www.packtpub.com/free-eBook/xamarin-cross-platform-development-cookbook) - George Taskos, Packt. (Just fill the fields with any values)