flatpak: Add guidance on using ci-flatdeb-builder

https://phabricator.apertis.org/T8509

Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com

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This is in a draft state pending the actual ci-flatdeb-builder repository creation & the resulting filling of the TODOs in these changes, but other than that, this should be essentially ready. (Would like feedback here first, actually, on the configuration file format and such.)