releaso-notes: Document changes from #7349.

Thanks to @domenkozar for implicitly reminding me that documentation is
probably our biggest issue. And I'm a dumbass for contributing to that
situation, so let's do better than that and document it.

The current changes are only preparation for a bigger change coming real
soon[TM] in Hydra and release-tools, so right now it's still a bit
tedious to create custom channels.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>