add doc about env format and docker backend

README.md

@@ -60,16 +60,20 @@ To test against a local hydra instance
  ```bash
  make start
  ```
+- You can also set your preset frontend URLs with environment variables, making them available in the admin UI. This allows you to switch between them seamlessly:
+ ```bash
+ RAZZLE_DEFAULT_IFRAME_URL=http://localhost:3002,https://hydra-blogsite-nextjs.vercel.app pnpm start
+ ```
+ You can find `.env.example` at root directory of the project.
 - Ensure Volto is running on port 3000.
 
 **3. Start the Plone Backend**
 
-- You can start the Plone backend using Docker images, or you can navigate to the core/api directory in Volto-Hydra and run:
+- You can start the Plone backend using Docker images:
  ```bash
- cd core/api
- make start
+ make backend-docker-start
  ```
-
+ ***Note :*** This will also set `CORS_ALLOW_ORIGIN` to `'*'`, so there are no cors error.
 ### Deploy your frontend
 
 Use netlify or similar and make your frontend public and then let us know by creating a ticket and we will advertise your frontend