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