nix: add nix documentation

Documentation/Nix-NixOS.md

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+# Getting started with NixOS on Droidspaces
+
+To build a minimal tarball archive to be used with Droidspaces, run:
+```sh
+nix build github:ravindu644/Droidspaces-OSS#nixosDroidspacesTarballs.aarch64-linux.minimal
+```
+
+If your device has a kernel version 5.4 or below, use `minimal-with-systemd-v259` instead. (Systemd v260 and above have dropped support for kernel 5.4 and below.)
+
+```sh
+nix build github:ravindu644/Droidspaces-OSS#nixosDroidspacesTarballs.aarch64-linux.minimal-with-systemd-v259
+```
+
+If the container boots, you can proceed to configuring NixOS for Droidspaces.
+
+
+# Configuring NixOS for Droidspaces
+
+To run NixOS on Droidspaces smoothly, import the `working-droidspaces-rootfs-minimal` module into your NixOS system.
+
+```nix
+# flake.nix
+droidspaces.url = "github:ravindu644/Droidspaces-OSS";
+
+# configuration
+{inputs, ...}: {
+  imports = [inputs.droidspaces.nixosModules.working-droidspaces-rootfs-minimal];
+}
+```
+
+**NOTE:** As previously mentioned, kernel 5.4 and below will not be able to run NixOS systems from newer nixpkgs, so use a pinned nixpkgs version:
+```nix
+nixpkgs-with-systemd-v259.url = "github:NixOS/nixpkgs/b86751bc4085f48661017fa226dee99fab6c651b";
+```
+
+Adding this module will also let you build your system as a tarball archive:
+```sh
+nix build .#<hostname>.config.system.build.tarball
+```
+
+
+# Systemd issues on older kernels
+
+Newer systemd versions may have trouble running on older kernels. You may need to find and use an older nixpkgs release that still supports your kernel.
+
+
+# NixOS without systemd
+
+If you don't want to run systemd, you can change the init path in Droidspaces to `/bin/sh`. This will allow you to enter the container normally, but systemd services will not be running.
+
+
+# Finix (no more systemd) (experimental)
+
+Finix is an experimental NixOS-like system running finit instead of systemd.
+
+To build a Finix tarball:
+```sh
+nix build github:ravindu644/Droidspaces-OSS#finixDroidspacesTarballs.aarch64-linux.experimental
+```