Added "Learning the elixir language" ebook (#6573)

* Added "Learning the elixir language" ebook

I didn't see this in the list or pending pull requests. Thanks for maintaining this- I am excited about using it now that I know it exists.

* pdf only for riptorial

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

Co-authored-by: Eric Hellman <eric@hellman.net>
Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

books/free-programming-books-langs.md

@@ -659,6 +659,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Getting Started Guide](http://elixir-lang.org/getting-started/introduction.html) (HTML) [(PDF, MOBI, EPUB)](https://github.com/potatogopher/elixir-getting-started)
 * [Joy of Elixir](https://joyofelixir.com) - Ryan Bigg (HTML) - [Source](https://github.com/radar/joyofelixir) (:construction: *in process*)
 * [Learning Elixir](http://learningelixir.joekain.com) - Joseph Kain Blog (HTML)
+* [Learning the Elixir Language](https://riptutorial.com/Download/elixir-language.pdf) - Compiled from StackOverflow Documentation (PDF) 
 * [The Ultimate Guide To Elixir For Object-Oriented Programmers](http://www.binarywebpark.com/ultimate-guide-elixir-object-oriented-programmers) - Bruce Park (HTML)