model documentation

app/models/invoice.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # Invoice correspond to a single purchase made by an user. This purchase may
 # include reservation(s) and/or a subscription
 class Invoice < ActiveRecord::Base

app/models/invoice_item.rb

@@ -1,3 +1,6 @@
+# frozen_string_literal: true
+
+# A single line inside an invoice. Can be a subscription or a reservation
 class InvoiceItem < ActiveRecord::Base
   belongs_to :invoice
   belongs_to :subscription