Add description field to channels

[Sergio Schvezov]

Each version of an image has a description field, it would be good if a key like this could be added to the channels to add a human readable description of what the channel is about (or whatever the owner deems necessary) for fast identification.

With better channel management we could have maybe just avoided this, but there are too many customization channels around and it's hard to figure out what each one provides.