I know this has been discussed earlier, but I suggest to support xmp sidecar files for dng files as some cameras output their raw files as DNG's. Although the discussion earlier has been around whether the DNG metadata should be incorporated in the file or not (the standard seem to support both), the argument of not changing any raw files is to me a good argument. Adding metadata into a file is often an iterative process and may happen several times during the lifetime of a raw file, it feels dangerous to let any software change the raw files. In addition, each time a rating has been changed the whole dng file has changed which will also initiate a new backup for that file. Hence, a DNG file of 30-40 MB size will be backed up for any metadata changes, instead of an xmp file of a few KB size.
This is not something we're going to implement at this time. There may be some apps that create sidecars for DNG, but this is not actually the standard. XMP sidecar files would likely run into problems when using the photos with Adobe and other products along with problems with handling the sidecar when copying or deleting photos.