core: make note that the box is being added directly [GH-5311]

lib/vagrant/action/builtin/box_add.rb

@@ -115,6 +115,8 @@ module Vagrant
         # @param [Array<String>] urls
         # @param [Hash] env
         def add_direct(urls, env)
+          env[:ui].output(I18n.t("vagrant.box_adding_direct"))
+
           name = env[:box_name]
           if !name || name == ""
             raise Errors::BoxAddNameRequired

templates/locales/en.yml

@@ -20,6 +20,8 @@ en:
       Adding box '%{name}' (v%{version}) for provider: %{providers}
     box_added: |-
       Successfully added box '%{name}' (v%{version}) for '%{provider}'!
+    box_adding_direct: |-
+      Box file was not detected as metadata. Adding it directly...
     box_downloading: |-
       Downloading: %{url}
     box_download_error: |-