Document experimental bcachefs usage

[davidak]

how to use it?

see for reference:

https://github.com/davidak/nixos-config/tree/master/machines/install-iso-bcachefs

[stale]

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[davidak]

still needed

[Madouura]

I feel we do need some documentation about this, at least until it's mainlined.
It's not exactly hard to generate an iso with boot.supportedFilesystems = [ "bcachefs" ];, but that at least needs to be documented.