templates/show-swhid: Add link to HOWTO reference your code page

Related to T3908

swh/web/templates/includes/show-swhids.html

@@ -17,7 +17,12 @@ See top-level LICENSE file for more information
     {% endif %}
     <div id="swh-identifiers-content">
       <p>
-        To reference or cite the objects present in the Software Heritage archive, permalinks based on
+        To
+        <a target="_blank" rel="noopener noreferrer"
+           href="https://www.softwareheritage.org/howto-archive-and-reference-your-code/">
+          reference or cite
+        </a>
+        the objects present in the Software Heritage archive, permalinks based on
         <a target="_blank" rel="noopener noreferrer"
            href="https://docs.softwareheritage.org/devel/swh-model/persistent-identifiers.html">
           SoftWare Heritage persistent IDentifiers (SWHIDs)

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T3908 is now swh/meta#3908 (moved)