update for 8.6.0-beta2 release

Author: jmckenna

en/download.txt

@@ -68,6 +68,7 @@ Current Release
 Previous Releases
 ..............................................................................
 
+* `MapServer-8.4-Documentation.pdf <https://download.osgeo.org/mapserver/docs/MapServer-84.pdf>`__ |image|
 * `MapServer-8.2-Documentation.pdf <https://download.osgeo.org/mapserver/docs/MapServer-82.pdf>`__ |image|
 * `MapServer-8.0-Documentation.pdf <https://download.osgeo.org/mapserver/docs/MapServer-80.pdf>`__ |image|
 * `MapServer-7.6-Documentation.pdf <https://download.osgeo.org/mapserver/docs/MapServer-76.pdf>`__ |image|