Added Sorella's Understanding JS OOP article.

free-programming-books.md

@@ -1175,6 +1175,7 @@ Original Source: [Free Programming books](http://stackoverflow.com/revisions/392
 * [The JavaScript Tutorial](http://javascript.info/)
 * [The Problem with Native JavaScript APIs](http://chimera.labs.oreilly.com/books/1234000001655) (PDF)
 * [Understanding ECMAScript 6](https://leanpub.com/understandinges6/read) *(Leanpub, by Nicholas C. Zakas)*
+* [Understanding JavaScript OOP](http://robotlolita.me/2011/10/09/understanding-javascript-oop.html) *(Sorella)*
 * [You Don't Know JS](https://github.com/getify/You-Dont-Know-JS)