private.xml

<?xml version="1.0"?>
<root>
    <item>
    <name>Logitech K811 Previous/Forward Media Keys</name>
    <appendix>Make F14 and F15 (brightness) Prev/Forward media keys</appendix>
    <identifier>private.rjoc.k811_prev_forward</identifier>

    <device_only>DeviceVendor::LOGITECH, DeviceProduct::LOGITECH_K811</device_only>

    <autogen>
      __KeyToKey__
      KeyCode::F14,
      ConsumerKeyCode::MUSIC_PREV
    </autogen>
    <autogen>
      __KeyToKey__
      KeyCode::F15,
      ConsumerKeyCode::MUSIC_NEXT
    </autogen>
  </item>
</root>