Add Elixir Client to Readme

I've developed a full-fledged elixir client for Toxiproxy that is highly inspired by the ruby version.
It's called `ToxiproxyEx` but I think we can link it as `toxiproxy-elixir` to make it clear that it is an elixir client.

README.md

@@ -83,6 +83,7 @@ development and CI environments.
 * [toxiproxy-node-client](https://github.com/ihsw/toxiproxy-node-client)
 * [toxiproxy-java](https://github.com/trekawek/toxiproxy-java)
 * [toxiproxy-haskell](https://github.com/jpittis/toxiproxy-haskell)
+* [toxiproxy-elixir](https://github.com/Jcambass/toxiproxy_ex)
 
 ## Example