add since info on encoding attribute

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@450588 13f79535-47bb-0310-9956-ffa450edef68

docs/manual/CoreTasks/echo.html

@@ -68,7 +68,7 @@ ignored</p>
   </tr>
   <tr>
     <td valign="top">encoding</td>
-    <td valign="top">encoding to use, default is ""; the local system encoding.</td>
+    <td valign="top">encoding to use, default is ""; the local system encoding. <em>since Ant 1.7</em></td>
     <td valign="top" align="center">No</td>
   </tr>
 </table>