docs: add note about directory import

docs/7.migration/2.configuration.md

@@ -77,6 +77,17 @@ Nuxt and Nuxt Modules are now build-time-only.
 
 1. Move all your `buildModules` into `modules`.
 2. Check for Nuxt 3 compatibility of modules.
+3. If you have any local modules pointing to a directory you should update this to point to the entry file:
+
+```diff
+  export default defineNuxtConfig({
+    modules: {
+-     '~/modules/my-module'
++     '~/modules/my-module/index'
+    }
+  })
+```
+
 
 ::alert
 If you are a module author, you can check out [more information about module compatibility](/docs/migration/module-authors) and [our module author guide](/docs/guide/going-further/modules).