Added default api controller.

lib/beet_round_server_web/controllers/default_api_controller.ex

@@ -0,0 +1,7 @@
+defmodule BeetRoundServerWeb.DefaultApiController do
+  use BeetRoundServerWeb, :controller
+
+  def index(conn, _params) do
+    text(conn, "The Beet Round API (#{Mix.env()}) is running.")
+  end
+end