Add linux installation informations

[pprw]

Thanks for taking this wonderful software to a next level.

This is just a quick issue to suggest adding links to snap and flatpak packages. The installation for linux is not detailed and one could think compilation is the only way.

https://snapcraft.io/scantailor-advanced https://flathub.org/apps/details/com.github._4lex4.ScanTailor-Advanced

[leonleon33156]

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