Updates free-programming-books.md (#2568)

* Updates free-programming-books.md

Adds a link to a free image optimization book

* fixes failing build

removed trailing slash

* adds author name

free-programming-books.md

@@ -475,6 +475,7 @@
 * [Distributed systems for fun and profit](http://book.mixu.net/distsys/single-page.html)
 * [Docker Jumpstart](http://odewahn.github.io/docker-jumpstart/)
 * ["DYNAMIC LINKED LIBRARIES": Paradigms of the GPL license in contemporary software](http://www.lulu.com/shop/http://www.lulu.com/shop/luis-enr%C3%ADquez-a/dynamic-linked-libraries-paradigms-of-the-gpl-license-in-contemporary-software/ebook/product-21419788.html) - Luis A. Enríquez
+* [Essential Image Optimization](https://images.guide) - Addy Osmani
 * [Flow based Programming](http://jpaulmorrison.com/fbp/)
 * [Foundations of Programming](http://openmymind.net/FoundationsOfProgramming.pdf) (PDF)
 * [Game Programming Patterns](http://gameprogrammingpatterns.com) - Bob Nystrom