added rtmidi=jack info

overview/realtimelinux.xml

@@ -413,6 +413,15 @@ csound5:output2             (playback right)</screen>
 	The sample rate of the orchestra must be the same as that of the
 	JACK server.
       </para>
+            <para>
+	Jack can also be used for MIDI IO. For this -+rtmidi=jack is
+	needed. For input -M followed by the requested jack MIDI port name is
+	used to connect directly to an input stream. For output -Q
+	followed by the jack port name is used. The options
+	-+jack_midi_inportname= and -+jack_midi_outportname= can be
+	used to rename Csound's MIDI IO ports.
+      </para>
+
       <bridgehead>Using Pulseaudio</bridgehead>
       <para>Support for <ulink url="http://www.pulseaudio.org/">Pulseaudio</ulink> was added in Csound 5.09. You can specify the following settings:</para>
       <orderedlist>