docs: document new option type

README.md

@@ -308,7 +308,7 @@ FASTIFY_AUTOLOAD_TYPESCRIPT=1 node --loader=my-custom-loader index.ts
 
 Each plugin can be individually configured using the following module properties:
 
-- `plugin.autoConfig` - Configuration object which will be used as `opts` parameter
+- `plugin.autoConfig` - Specifies the options to be used as the `opts` parameter.
 
   ```js
   module.exports = function (fastify, opts, next) {
@@ -332,6 +332,13 @@ Each plugin can be individually configured using the following module properties
   export const autoConfig = { name: 'y' }
   ```
 
+  You can also use a callback function if you need to access the parent instance:
+  ```js
+  export const autoConfig = (fastify) => {
+    return { name: 'y ' + fastify.rootName }
+  }
+  ```
+
 - `plugin.autoPrefix` - Set routing prefix for plugin
 
   ```js