manually added hmac how-to link to get.html.erb

Author: meihDS

app/views/connect/cneg001_validate_webhook_message/get.html.erb

@@ -1,5 +1,9 @@
 <%= render('partials/example_info') %>
 
+<p>
+  Prerequisite: See <a target='_blank' href="https://developers.docusign.com/platform/webhooks/connect/validate/">How to validate an HMAC signature</a>.
+</p>
+
 <% form_index = 0 %>
 <% secret_index = 0 %>
 <% payload_index = 1 %>