Note not to put rtproto in ns fils for Windows.

www/doc/windows_in_emulab_user.html

@@ -118,6 +118,9 @@ Currently available images are:
 
 </ul>
 
+Note: routing is not currently supported on Windows.  Don't put any
+<b>rtproto</b> statements in your .ns file.  <p>
+
 Notice that Windows reboots an extra time after first swap-in on a node, after
 changing the node name, to set up the network stack properly.   <p>