docs: add info about layers/ directory (nuxt#28128)

docs/1.getting-started/9.layers.md

@@ -18,7 +18,11 @@ One of the core features of Nuxt is the layers and extending support. You can ex
 
 ## Usage
 
-You can extend a layer by adding the [extends](/docs/api/nuxt-config#extends) property to the [`nuxt.config.ts`](/docs/guide/directory-structure/nuxt-config) file.
+By default, any layers within your project in the `~/layers` directory will be automatically registered as layers in your project
+
+::note Layer auto-registration was introduced in Nuxt v3.12.0 ::
+
+In addition, you can extend from a layer by adding the [extends](/docs/api/nuxt-config#extends) property to your [`nuxt.config`](/docs/guide/directory-structure/nuxt-config) file.
 
 ```ts [nuxt.config.ts]
 export default defineNuxtConfig({