example: Add example of user authencate l2 switch like an AP on cafe

examples/cafe/mix.exs

@@ -0,0 +1,20 @@
+defmodule Cafe.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      apps_path: "apps",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Dependencies listed here are available only for this
+  # project and cannot be accessed from applications inside
+  # the apps folder.
+  #
+  # Run "mix help deps" for examples and options.
+  defp deps do
+    []
+  end
+end