Add Easy Rust in Rust section. (EbookFoundation#6718)

* Add Easy Rust in Rust section.

* Update books/free-programming-books-langs.md

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

Author: zz-xx

books/free-programming-books-langs.md

@@ -2150,6 +2150,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 
 * [A Gentle Introduction To Rust](https://stevedonovan.github.io/rust-gentle-intro) - Steve J Donovan
 * [Asynchronous Programming in Rust](https://rust-lang.github.io/async-book)
+* [Easy Rust](https://dhghomon.github.io/easy_rust/) - David McLeod (HTML, PDF)
 * [Guide to Rustc Development](https://rustc-dev-guide.rust-lang.org)
 * [Learn Rust With Entirely Too Many Linked Lists](https://rust-unofficial.github.io/too-many-lists) - Alexis Beingessner
 * [Rust by Example](https://doc.rust-lang.org/stable/rust-by-example)