Add a section on upgrading to the general documentation, a=chris

diff --git a/en-US/pages/documentation.thtml b/en-US/pages/documentation.thtml
index 208fd18..178a866 100755
--- a/en-US/pages/documentation.thtml
+++ b/en-US/pages/documentation.thtml
@@ -7,6 +7,7 @@
         <li><a href="#features">Feature List</a></li>
         <li><a href="#requirements">Requirements</a></li>
         <li><a href="#installation">Installation and Configuration</a></li>
+        <li><a href="#upgrade">Upgrading Yioop</a></li>
         <li><a href="#files">Summary of Files and Folders</a></li>
         <li><a href="#interface">Yioop Search and User Interface</a></li>
         <li><a href="#mobile">Yioop Mobile Interface</a></li>
@@ -725,6 +726,17 @@ After filling in all the fieldsets and submitting the form,
 the installation is complete.
 </p>
     <p><a href="#toc">Return to table of contents</a>.</p>
+
+<h2 id='upgrade'>Upgrading Yioop</h2>
+<p>If you have an older version of Yioop that you would like to upgrade,
+ make sure to back up your data. Then download the latest
+version of Yioop and unzip it to the location you would like. Set the
+Search Engine Work Directory by the same method and to the same value as
+your old Yioop Installation. See the Installation section above for
+instructions on this, if you have forgotten how you did this.
+Knowing the old Work Directory location, should
+allow Yioop to complete the upgrade process.</p>
+    <p><a href="#toc">Return to table of contents</a>.</p>
     <h2 id='files'>Summary of Files and Folders</h2>
     <p>The Yioop search engine consists of three main
 scripts:</p>