Add link of official SOGo wiki tutorial for restore and backup.

en_US/faq/backup.restore.md

@@ -60,6 +60,8 @@ Notes:
   document to generate DKIM key and update DNS record:
   [Sign DKIM signature on outgoing emails for new mail domain](./sign.dkim.signature.for.new.domain.html).
 
+* SOGo (Calendar, Contacts, Tasks): check SOGo official document:
+  <http://wiki.sogo.nu/backupRestore>
 * OpenLDAP backend:
 
     * If you enabled additional LDAP schema files in OpenLDAP, you should

html/backup.restore.html

@@ -90,6 +90,10 @@ defined as cron jobs with below commands:</p>
   <a href="./sign.dkim.signature.for.new.domain.html">Sign DKIM signature on outgoing emails for new mail domain</a>.</p>
 </li>
 <li>
+<p>SOGo (Calendar, Contacts, Tasks): check SOGo official document:
+  <a href="http://wiki.sogo.nu/backupRestore">http://wiki.sogo.nu/backupRestore</a></p>
+</li>
+<li>
 <p>OpenLDAP backend:</p>
 <ul>
 <li>If you enabled additional LDAP schema files in OpenLDAP, you should