Update free-programming-books.md

Add EPUB, PDF, MOBI for Elixir's Getting Started tutorial

free-programming-books.md

@@ -975,6 +975,7 @@
 * [Elixir School](https://elixirschool.com) (HTML)
 * [Études for Elixir](http://chimera.labs.oreilly.com/books/1234000001642/index.html) (HTML)
 * [Getting Started Guide](http://elixir-lang.org/getting-started/introduction.html) (HTML)
+* [Getting Started Guide](https://github.com/potatogopher/elixir-getting-started) (PDF, MOBI, EPUB)
 * [Learning Elixir](http://learningelixir.joekain.com) - Joseph Kain Blog (HTML)