         import_config config
       end
 
+  ## config/releases.exs
+
+  If you are using releases, see `mix release`, there another configuration
+  file called `config/releases.exs`. While `config/config.exs` and friends
+  mentioned in the previous section are executed whenever you run a Mix
+  command, including when you assemble a release, `config/releases.exs` is
+  execute every time your production system boots. Since Mix is not available
+  in a production system, `config/releases.exs` must not use any of the
+  functions from Mix.
   """
 
   @config_key {__MODULE__, :config}

  `import Config` will import the functions `config/2`, `config/3`
  and `import_config/1` to help you manage your configuration.

  Once Mix starts, it will automatically evaluate the configuration
  file and persist it into `:some_app`'s application environment, which
  can be accessed in as follows:

      "value1" = Application.fetch_env!(:some_app, :key1)

  See `Config.Reader` for evaluating and reading configuration
  files.

  **Important:** if you are writing a library to be used by other developers,
  it is generally recommended to avoid the application environment, as the


  ## Example

  This module is most commonly used to define application configuration,
  typically in `config/config.exs`:

      import Config

