add free ebook notification

README.md

@@ -36,3 +36,7 @@ knowing the keyword is required to even know what to search for.
 * [Rust Cookbook](https://www.packtpub.com/application-development/rust-cookbook?utm_source=github&utm_medium=repository&utm_campaign=9781785880254)
 
 
+### Download a free PDF
+
+ <i>If you have already purchased a print or Kindle version of this book, you can get a DRM-free PDF version at no cost.<br>Simply click on the link to claim your free PDF.</i>
+<p align="center"> <a href="https://packt.link/free-ebook/9781788839358">https://packt.link/free-ebook/9781788839358 </a> </p>