Add experimental warning note (#41666)

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

packages/next/build/webpack-config.ts

@@ -593,6 +593,11 @@ export default async function getBaseWebpackConfig(
         'You are using the experimental Node.js Runtime with `experimental.runtime`.'
       )
     }
+    if (config.experimental.appDir) {
+      Log.warn(
+        'You are using the experimental app directory with `experimental.appDir`, the API might change.'
+      )
+    }
   }
 
   const babelConfigFile = await BABEL_CONFIG_FILES.reduce(