Add ex_doc as a dependency

mix.exs

@@ -23,6 +23,7 @@ defmodule Aife.MixProject do
     [
       # {:dep_from_hexpm, "~> 0.3.0"},
       # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+      {:ex_doc, "~> 0.19", only: :dev, runtime: false},
     ]
   end
 end