ci: Shellcheck stuff needs documenting

[tcharding]

We just started adding a shellcheck CI job all over the place, it requires specific code comments to remove false positives. We should add these to the ci/README.md.

[apoelstra]

It would be nicer if we could somehow change the way we source stuff so we wouldn't need the warnings.

As always, it would be nice to switch away from shell, but IIRC we have never found a suitable replacement language that everybody can tolerate (and the chatbot has made shell much more tolerable..).