message: Add link to preprocessing WG Slack channel
diff: @@ -18,6 +18,7 @@ You can find the schedule of meetings on the [LF AI wiki](https://wiki.lfai.foun
  | ------------------ | ------------- |
  | [Release](https://lfaifoundation.slack.com/archives/C018VGGJUGK) | Improve the release process for ONNX |
  | [Training](https://lfaifoundation.slack.com/archives/C018K560U14) | Expand ONNX to support training as well as inference |
  +| [Preprocessing](https://lfaifoundation.slack.com/archives/C02AANGFBJB) | Make data preprocessing part of ONNX |
  ## Completed working groups
  