nix machine / user configurations

Merge pull request #297 from Narice/core

Added note about cache in start/index.md docs

Pacman99 f88acc16 a81930af

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start
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doc/start/index.md
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> files before building the system.
> - You can choose to simply clone the repo with git if you want to follow
> upstream changes.
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> - If the `nix-shell -p cachix --run "cachix use nrdxp"` line doesn't work
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> you can try with sudo: `sudo nix-shell -p cachix --run "cachix use nrdxp"`
## Next Steps:
- [Make installable ISO](./iso.md)