You can use volume automation on your output track.
1) Open the mixer and click on the button just above the pan knob on the Output strip. Select "Latch."
2) As soon as you do this, the Output track will appear in the arrange window below your other tracks.
3) Press "A" to show automation on your tracks.
4) Press on the long gray bar beneath the Output track name and select "Volume."
5) Click anywhere on the yellow automation line to create two nodes and drag the last one down for your fade.
That should do it.