Add warning message for supplemental content

Refs #445

lib/govuk_design_system_formbuilder/containers/supplemental.rb

@@ -10,6 +10,8 @@ def initialize(builder, object_name, attribute_name, content)
       def html
         return if @content.blank?
 
+        warn("Supplemental content is deprecated and support will soon be removed. See https://github.com/x-govuk/govuk-form-builder/issues/445")
+
         tag.div(id: supplemental_id) { @content }
       end
 

spec/support/shared/shared_block_examples.rb

@@ -25,6 +25,15 @@
         with_tag('p', text: block_p)
       end
     end
+
+    describe "deprecation warning message" do
+      before { allow(Rails).to receive_message_chain(:logger, :warn).with(any_args).and_return(true) }
+
+      specify 'logs a deprecation warning' do
+        subject
+        expect(Rails.logger).to have_received(:warn).with(/Supplemental content is deprecated/)
+      end
+    end
   end
 
   context 'when no block is supplied' do