Add URL slug help

app/views/posts/new_help.html.erb

@@ -26,6 +26,7 @@
   </div>
   <div class="form-group">
     <%= f.label :doc_slug, 'URL slug', class: "form-element" %>
+    <span class="form-caption">In a URL of "https://yoursite.codidact.com/help/topic", the "topic" is the slug.</span>
     <%= f.text_field :doc_slug, class: 'form-element' %>
   </div>
   <div class="form-group">