Fix README to allow test as NODE_ENV (#1722)

`NODE_ENV=test` is allowed by rails/webpacker#1563.

README.md

@@ -230,7 +230,7 @@ staging:
   public_output_path: packs-staging
 ```
 
-or, Webpacker will use production environment as a fallback environment for loading configurations. Please note, `NODE_ENV` can either be set to `production` or `development`.
+or, Webpacker will use production environment as a fallback environment for loading configurations. Please note, `NODE_ENV` can either be set to `production`, `development` or `test`.
 This means you don't need to create additional environment files inside `config/webpacker/*` and instead use webpacker.yml to load different configurations using `RAILS_ENV`.
 
 For example, the below command will compile assets in production mode but will use staging configurations from `config/webpacker.yml` if available or use fallback production environment configuration: