Add link to exercism.io

I've done a couple of the Elixir challenges on exercism.io and found them quite helpful. A lot of resources for learning Elixir mention them as being helpful too (such as this talk from Cameron Price, and the article on learning Elixir on Made By Many) , I think a link at the bottom to this README with a link could be welcome addition to this repo to give the person learning a place to flex their new Elixir muscles with some challenges.