Added a book on C++ (#2131)

free-programming-books.md

@@ -859,6 +859,7 @@
 * [Financial Numerical Recipes in C++](http://finance.bi.no/~bernt/gcc_prog/recipes/) - Bernt Arne Ødegaard
 * [Game Programming Patterns](http://gameprogrammingpatterns.com)
 * [Google's C++ Style Guide](https://google.github.io/styleguide/cppguide.html)
+* [How to make an Operating System](https://www.gitbook.com/book/samypesse/how-to-create-an-operating-system/details) - Samy Pesse
 * [How To Think Like a Computer Scientist: C++ Version](http://greenteapress.com/thinkcpp/index.html) - Allen B. Downey
 * [Introduction to Design Patterns in C++ with Qt](http://ptgmedia.pearsoncmg.com/images/9780131879058/downloads/0131879057_Ezust_book.pdf) (PDF)
 * [Joint Strike Fighter, C++ Coding Standards, by Bjarne Stroustrup](http://www.stroustrup.com/JSF-AV-rules.pdf) (PDF)