Add guidance to billing report page

Following a team cuddle where we checked how different people
interpret the billing report. We found that some things about
the report are ambiguous and that it would be helpful to have
some guidance on the page where the report can be downloaded,

app/templates/views/platform-admin/get-billing-report.html

@@ -17,4 +17,22 @@
     {{ page_footer('Download report') }}
   {% endcall %}
 
+  <h2 class="heading-medium">
+    Guidance for interpreting the report
+  </h2>
+  <p class="govuk-body">
+    Billing report shows data for period between start date and end date, inclusive of both dates.
+  </p>
+  <p class="govuk-body">
+    Explanation for more confusing column names in the report:
+  </p>
+  <ul class="govuk-list govuk-list--bullet">
+    <li>sms cost - cost accrued by a service through sending text messages after they run out of free allowance</li>
+    <li>sms chargeable units - sms fragments that go over free allowance multiplied by rate multiplier (so higher costs of sending international SMS are taken into consideration)</li>
+    <li>letter cost - cost accrued by a service through sending letters (number of letters * numbers of paper sheets * postage)</li>
+    <li>letter breakdown - amount and cost of letters sent by a service for each postage class</li>
+    <li> purchase order number, contact names, contact email addresses, billing reference - this data is manually filled in by us based on the information we get from services. You can help by filling it in (on Service Settings page)</li>
+
+  </ul>
+
 {% endblock %}