Improve update instructions (drupal-composer#366)

Author: alexpott

README.md

@@ -61,7 +61,7 @@ new release of Drupal core.
 
 Follow the steps below to update your core files.
 
-1. Run `composer update drupal/core --with-dependencies` to update Drupal Core and its dependencies.
+1. Run `composer update drupal/core webflo/drupal-core-require-dev symfony/* --with-dependencies` to update Drupal Core and its dependencies.
 1. Run `git diff` to determine if any of the scaffolding files have changed. 
    Review the files for any changes and restore any customizations to 
   `.htaccess` or `robots.txt`.