docs(zod-openapi): update readme

packages/zod-openapi/README.md

@@ -4,10 +4,6 @@
 
 _Note: This is not standalone middleware but is hosted on the monorepo "[github.com/honojs/middleware](https://github.com/honojs/middleware)"._
 
-## Limitations
-
-- An instance of Zod OpenAPI Hono cannot be used as a "subApp" in conjunction with `rootApp.route('/api', subApp)`.
-
 ## Usage
 
 ### Installation
@@ -208,6 +204,47 @@ const appRoutes = app.openapi(route, (c) => {
 const client = hc<typeof appRoutes>('http://localhost:8787/')
 ```
 
+## Limitations
+
+An instance of Zod OpenAPI Hono cannot be used as a "subApp" in conjunction with `rootApp.route('/api', subApp)`.
+Use `app.mount('/api', subApp.fetch)` instead.
+
+```ts
+const api = OpenAPIHono()
+
+// ...
+
+// Set the `/api` as a base path in the document.
+api.get('/doc', (c) => {
+  const url = new URL(c.req.url)
+  url.pathname = '/api'
+  url.search = ''
+
+  return c.json(
+    // `api.getOpenAPIDocument()` will return a JSON object of the docs.
+    api.getOpenAPIDocument({
+      openapi: '3.0.0',
+      info: {
+        version: '1.0.0',
+        title: 'My API',
+      },
+      servers: [
+        {
+          url: `${url.toString()}`,
+        },
+      ],
+    })
+  )
+})
+
+const app = new Hono()
+
+// Mount the Open API app to `/api` in the main app.
+app.mount('/api', api.fetch)
+
+export default app
+```
+
 ## References
 
 - [Hono](https://hono.dev/)