faq for compat bounds for jll packages

[Krastanov]

Based on this discussion https://discourse.julialang.org/t/why-do-packages-run-continuous-integration-tests-on-julia-nightly/101208/41?u=krastanov

It seems there is lack of clarity in the community on how to deal with jll builds of packages that do not follow semver.

I tried to convey what @giordano was describing, but please double check what I have written.

[Krastanov]

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