fix(docs): add link to ebook on Gumroad

Author: MinaPecheux

README.md

@@ -4,6 +4,8 @@
 
 ![thumbnail](imgs/thumbnail.jpg)
 
+### 📕 Discover the series as an ebook and get bonus material <a rel="noopener follow" href="https://mpecheux.gumroad.com/l/rrylr"><strong>on Gumroad</strong></a>!
+
 This repository contains the code for my on-going series of tutorials on how to make a real-time strategy (RTS) game in the well-known game engine Unity! Throughout this series of tutorials, I explore C# scripting for games, GUI building, event systems, behavior trees, sound optimization...
 
 ![demo](imgs/demo.gif)