Added section about upgrade and the database structure change

documentation/installation_guide.html

@@ -305,6 +305,13 @@ previous version of Chamilo LMS (1.*) or Dok€os (<2.0)</h2>
 Before upgrading, we <b>heavily</b> recommend you do a full backup of the previous
 Chamilo or Dok€os directories and databases. If you are unsure how to achieve this
 please ask your hosting provider for advice.<br />
+<em>Chamilo LMS 1.9 comes with a new database structure. Although the upgrade script
+takes the migration in charge, it will be generate a very heavy load on your server
+during the upgrade, and will change your database completely, preserving the data
+as well as possible (we have tested the procedure many times, but remember this is
+GNU/GPLv3 and we cannot be held responsible for what would happen to your data
+without professional supervision). This is why we *really* recommend you take
+a full backup of your system before you upgrade.</em>
 <br />
 <div class="muted"> NOTE: This version of Chamilo can only be used to upgrade from 
 smaller versions of Chamilo or Dok€os. For example, you cannot use the normal