Added a related link from CSS placeholder instructions to the possibi…

…lity to use a PI in new document templates to specify a placeholder content.

DITA/topics/dg-placeholder-css-extension.dita

@@ -53,4 +53,9 @@
     <note>Deprecated properties <codeph>show-placeholder</codeph> and
         <codeph>placeholder-content</codeph> are also supported.</note>
   </body>
+  <related-links>
+    <link href="customizing-templates.dita#customizing-templates">
+      <linktext>Using placeholders in document templates</linktext>
+    </link>
+  </related-links>
 </topic>