Update readme to use recipes/debian.yml consistently

Arnaud Rebilloutrequested to merge
arnaudr/pipeline:readme into master

The file recipes/debian.yml was added back in 2020 in commit ab8f0796. The "official way" to enable Salsa CI pipelines is to include this file, as documented in the TLDR.

Yet the readme was never updated to reflect that. Fixed with this commit.

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