docs: add document explaining differences

DIFFERENCES_NIXOS.md

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+# Differences between vanilla NixOS and nix-unify
+
+## Systemd services
+
+Services aren't automatically enabled
+
+`nix-unify.modules.shareSystemd.enable` (default true) must be enabled for systemd sharing to work
+
+In nix-unify, after creating a service it needs to be added to `nix-unify.modules.shareSystemd.units`
+
+In order to override an existing host-service the service additionally needs to be added to `nix-unify.modules.shareSystemd.replace`
+
+## Path
+
+`nix-unify.modules.sharePath.enable` (default true) must be enabled for /run/current-system/sw/bin to be added to the host's path
+
+## Networkd
+
+`nix-unify.modules.shareNetworkd.enable` (default false) must be enavbled for networkd configuration to be replaced by the one provided by nixos
+
+Note: If ifupdown is detected, ifupdown is disabled before activating networkd.
+
+## Tmpfiles
+
+Not currently supported, support planned
+
+## /etc
+
+Symlinks in /etc aren't automatically created
+
+`nix-unify.files.etc."file-or-folder" = {}` can be used to create a symlink from the nixos /etc to the host's /etc
+
+## Starting services
+
+For now this needs to be done manually or via rebooting. Support wip.