chore: add more info to migration (#723)

* chore: add more info to migration

Add info about removing `node_bundler` and `external_node_modules` configs.

* Update README.md

Author: ascorbic

README.md

@@ -31,7 +31,7 @@ publish = ".next"
 package = "@netlify/plugin-nextjs"
 ```
 
-If you previously set `target: "serverless"` in your `next.config.js` this is no longer needed and can be removed.
+If you previously set `target: "serverless"`, `node_bundler` or `external_node_modules` in your `next.config.js` these are longer needed and can be removed.
 
 ## Beta feedback