Add another Nim book to this list. (#5482)

* Add another Nim book to this list.

Add the Computer Programming with the Nim Programming Language book to this list.

* Update books/free-programming-books.md
Co-authored-by: NDavid Ordás <davorpatech@gmail.com>

* Update books/free-programming-books.md
Co-authored-by: NDavid Ordás <davorpatech@gmail.com>
Co-authored-by: NDavid Ordás <davorpatech@gmail.com>

books/free-programming-books.md

@@ -2207,6 +2207,7 @@ Kerridge (PDF) (email address *requested*, not required)
 
 ### Nim
 
+* [Computer Programming with the Nim Programming Language](http://ssalewski.de/nimprogramming.html) - Dr. Stefan Salewski
 * [Nim Basics](https://narimiran.github.io/nim-basics) - narimiran
 * [Nim by Example](https://nim-by-example.github.io) - Flaviu Tamas
 * [Nim Days](https://xmonader.github.io/nimdays) - Ahmed Thabet