docs: Update migrate-config README (#45)

Author: nzakas

packages/migrate-config/README.md

@@ -42,9 +42,11 @@ bunx @eslint/migrate-config .eslintrc.json --commonjs
 
 Once you have completed the migration, you may need to manually modify the resulting config file.
 
-### Double-Check Compatibility
+### Upgrade Plugins and Double-Check Compatibility
 
-There are some plugins that might be used with compatibility features in the output that may no longer need them. You should double-check your plugins to see if they have fully ESLint v9-compatible versions of the packages.
+For any plugins used in your configuration file, make sure to upgrade to the latest version. Many plugins are creating their v9-compatible versions and this tool may be out-of-date in relation to which plugins fully support v9.
+
+After upgrading, double-check if the plugin still needs any of the compatibility utilities that were autogenerated as part of the migration.
 
 ### `--ext`