Add mix down export to 1.3 changelog (#3173)

CHANGELOG.md

@@ -33,6 +33,9 @@
     - Press and hold `RECORD` + turn  `SELECT`
     - Enter the Song menu while in Song or Arranger View by pressing `SELECT` and entering the `SONG > THRESHOLD RECORDING (THRE) > MODE` submenu
 
+#### <ins>Audio Export</ins>
+- Added `EXPORT MIXDOWN` configuration option which provides the ability to export all unmuted tracks in Arranger View as a single stereo file. This is disabled by default but can be enabled in the export configuration menu located at: `SONG\EXPORT AUDIO\CONFIGURE EXPORT\EXPORT MIXDOWN`
+
 #### <ins>Arranger View</ins>
 - Added ability to start / restart arrangement playback from the clip pad you're holding in arranger.
   - Note: you need to select a pad of any clip in arranger in order for this to work (it cannot be an empty pad)