Add installation guide section for Sendfile header - refs BT#6853

documentation/installation_guide.html

@@ -60,6 +60,7 @@
   <li><a href="#11._Chamilo_rapid_ppt_conversion">Chamilo Rapid - PPT conversion system</a></li>
   <li><a href="#12._Setting_cron_up">Setting chronological tasks</a></li>
   <li><a href="#13._Changing_language_name_order">Changing the language's firstname/lastname order</a></li>
+  <li><a hreg="#14._Improving_files_download">Improving files download time</a></li>
 </ol>
 
 <br />
@@ -820,6 +821,18 @@ Feel free to change this to<br />
 for example. The effect should be immediate.
 </p>
 <hr style="width: 100%; height: 2px;" />
+<h2><a name="14._Improving_files_download"></a>Improving files download efficiency</h2>
+<p>
+File download can be very slow when passing through a PHP script to control permissions. One solution to this
+    is to use the X-Sendfile header, which depends on a module on the webserver. <a href="http://stackoverflow.com/a/3731639/1406662">Check http://stackoverflow.com/a/3731639/1406662 for more details on implementing Sendfile</a>.
+Chamilo LMS 1.9.8 (and following versions) supports the X-Sendfile headers, but requires a specific line of configuration to be
+    added to configuration.php:
+<pre>
+$_configuration['enable_x_sendfile_headers'] = true;
+</pre>
+If you have issues with files taking a long time to download, make sure you reconfigure your webserver and add this line. You should see an notable difference in download time.
+</p>
+<hr style="width: 100%; height: 2px;" />
 <p>
   <br />
   <br />