Add introduction

app/templates/views/pricing/how-to-pay.html

@@ -9,12 +9,16 @@
 
     {{ page_header('How to pay') }}
 
+    <p class="govuk-body">
+      If you use GOV.UK Notify to send text messages or letters, you may need to send us a <a class="govuk-link govuk-link--no-visited-state" href="#purchase-orders">purchase order</a>.
+    </p>
+
     <h2 class="heading-medium" id="invoices">
       Invoices
     </h2>
 
     <p class="govuk-body">
-      GOV.UK Notify is run by the Government Digital Service (GDS), part of the Cabinet Office.
+      Notify is run by the Government Digital Service (GDS), part of the Cabinet Office.
     </p>
 
     <p class="govuk-body">