Added guide for `config.autoloader` [skip ci]

guides/source/configuring.md

@@ -157,6 +157,12 @@ defaults to `:debug` for all environments. The available log levels are: `:debug
 
 * `config.time_zone` sets the default time zone for the application and enables time zone awareness for Active Record.
 
+* `config.autoloader` sets the autoloading modes. This option defaults to `:zeitwerk` if `6.0` is specified in `config.load_defaults`. Can use the classic autoloader by specifying `:classic` after the line that loads` load_defaults `.
+
+    ```ruby
+    config.autoloader = :classic
+    ```
+
 ### Configuring Assets
 
 * `config.assets.enabled` a flag that controls whether the asset