docs: update README with rewrites feature support

Author: dphang

README.md

@@ -71,7 +71,7 @@ The following shows all supported features or planned features. Is there a featu
 - [x] [Preview mode](https://nextjs.org/docs/advanced-features/preview-mode)
 - [ ] [Optional catch all routes](https://nextjs.org/docs/routing/dynamic-routes#optional-catch-all-routes)
 - [x] [Redirects](https://nextjs.org/docs/api-reference/next.config.js/redirects). In latest alpha release now (caveat: every route should be able to redirect except `_next/static/*` and `static/*`, since those cache behaviors do not have Lambda handlers attached to them). See [RFC](https://github.com/serverless-nextjs/serverless-next.js/issues/587) for updates.
-- [ ] [Rewrites](https://nextjs.org/docs/api-reference/next.config.js/rewrites). See [RFC](https://github.com/serverless-nextjs/serverless-next.js/issues/587) for updates.
+- [x] [Rewrites](https://nextjs.org/docs/api-reference/next.config.js/rewrites). In latest alpha release now (caveats: every route should be able to rewrite except `_next/static/*` and `static/*`, since those cache behaviors do not have Lambda handlers attached to them. [External URL rewrites](https://nextjs.org/docs/api-reference/next.config.js/rewrites#rewriting-to-an-external-url) are not yet implemented).  See [RFC](https://github.com/serverless-nextjs/serverless-next.js/issues/587) for updates.
 - [ ] [Custom Headers](https://nextjs.org/docs/api-reference/next.config.js/headers). See [RFC](https://github.com/serverless-nextjs/serverless-next.js/issues/587) for updates.
 
 ### Getting started