Adding example input to editor categories (#1432)

* Adding example input to editor categories. Closes #1413

* Comma separated

Author: arfon

app/views/editors/_form.html.erb

@@ -83,8 +83,8 @@
     </div>
 
     <div class="col">
-      <%= f.label :category_list, "Categories (comma or space separated)" %>
-      <%= f.text_field :category_list, class: "form-control" %>
+      <%= f.label :category_list, "Categories (comma separated)" %>
+      <%= f.text_field :category_list, placeholder: "e.g., machine learning, image analysis, object detection", class: "form-control" %>
     </div>
   </div>
 </div>