Add JS book "Robust Client-Side Javascript" (#4837)

free-programming-books.md

@@ -1697,6 +1697,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Practical Modern JavaScript](https://github.com/mjavascript/practical-modern-javascript) - Nicolas Bevacqua
 * [Professor Frisby’s Mostly Adequate Guide to Functional Programming](https://mostly-adequate.gitbooks.io/mostly-adequate-guide/content/) - Brian Lonsdorf
 * [Programming JavaScript Applications](https://www.oreilly.com/library/view/programming-javascript-applications/9781491950289/)
+* [Robust Client-Side JavaScript](https://molily.de/robust-javascript/) - Matthias Schäfer (HTML, EPUB)
 * [Speaking JavaScript](http://speakingjs.com/es5/) - Dr. Axel Rauschmayer
 * [Standard ECMA-262 ECMAScript 2016 Language Specification](https://www.ecma-international.org/publications/standards/Ecma-262.htm) - Ecma International (HTML,PDF)
 * [The JavaScript Tutorial](http://javascript.info)