add `Code Simplicity: The Fundamentals of Software` by Max Kanat-Alexander (#6977)

Resolves #6976

books/free-programming-books-subjects.md

@@ -743,6 +743,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Architectural Styles and the Design of Network-based Software Architectures](http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm) - Roy Thomas Fielding
 * [Best Kept Secrets of Peer Code Review](http://smartbear.com/lp/ebook/collaborator/secrets-of-peer-code-review/)
 * [Building Secure & Reliable Systems](https://static.googleusercontent.com/media/landing.google.com/en//sre/static/pdf/Building_Secure_and_Reliable_Systems.pdf) - Heather Adkins, Betsy Beyer, Paul Blankinship, Piotr Lewandowski, Ana Oprea & Adam Stubblefield (PDF)
+* [Code Simplicity: The Fundamentals of Software](https://www.codesimplicity.com/book.pdf) - Max Kanat-Alexander (PDF)
 * [Data-Oriented Design](http://www.dataorienteddesign.com/dodmain/dodmain.html)
 * [Developing Reactive Microservices](https://info.lightbend.com/COLL-20XX-Developing-Reactive-Microservices_Landing-Page.html) (email address *requested*, not required)
 * [Domain Driven Design Quickly](http://www.infoq.com/minibooks/domain-driven-design-quickly)