document the interchangebility of the rating and color label

[UweOhse]

This is a pure documentation addendum.

I wasted a number of hours yesterday trying to figure out why my color labels didn't show up in another application, and wanted to spare others the same experience.

[elstoc]

I agree that this is useful information to have in the user manual. A couple of thoughts for consideration.

1. You talk about the information being "stored in a metadata field" but it's not clear what you mean by that. Do you mean "within the sidecar file" or "within an exported image" or both? The information is available in both but I'm guessing you mean the exported image?
2. You also talk about compatibility with other applications. This is a difficult one, and I'd be reluctant to pass comment on compatibility with anything outside of darktable, especially since it's not clear what we might mean by compatibility. For example, both of these fields can be queried by a tool that reads image metadata, and both are shown (albeit the color labels is a number so hard to interpret) when I view the image in geeqie. I could probably work out how to access the color label information in some other applications if I really wanted to. Perhaps we could word this better without speculating on other applications' functionality.
3. Finally, the rating is exported to Xmp.xmp.Rating, but from what I can see it's also exported to Exif.Image.Rating which is possibly a better field to reference.