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](https://www.youtube.com/watch?v=9uvp4h7gXHg), and [the article on learning Elixir on Made By Many](https://medium.com/the-many/learning-elixir-at-made-by-many-557737dafe55)) , 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.