docs: update version information in README (#141)

Author: chenjiahan

README.md

@@ -11,7 +11,7 @@ Use Rspack with a development server that provides live reloading. This should b
 
 ## Versions
 
-- `2.x`: For Rspack v2
+- `2.x`: For Rspack v2, see [v1 -> v2](./docs/migrate-v1-to-v2.md) for migration guide.
 - `1.x`: For Rspack v1, see [v1.x - README](https://github.com/rstackjs/rspack-dev-server/tree/v1.x#rspackdev-server) for usage guide.
 
 ## Installation