mix: add credo dependency

mix.exs

@@ -23,8 +23,9 @@ defmodule HttpSignatures.MixProject do
   # Run "mix help deps" to learn about dependencies.
   defp deps do
     [
-      # {:dep_from_hexpm, "~> 0.3.0"},
-      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"},
+      {:credo, "~> 1.0.0", only: [:dev, :test], runtime: false},
+      {:ex_doc, "~> 0.19", only: :dev, runtime: false},
+      {:dialyxir, "~> 1.0.0-rc.5", only: [:dev], runtime: false}
     ]
   end