add nix install instructions, thanks @siraben!

README.md

@@ -274,7 +274,7 @@ See the <a href="https://github.com/ArchiveBox/pip-archivebox"><code>pip-archive
 <ul>
 <li>Arch: <a href="https://aur.archlinux.org/packages/archivebox/"><code>pacman install archivebox</code></a> (contributed by <a href="https://github.com/imlonghao"><code>@imlonghao</code></a>)</li>
 <li>FreeBSD: <a href="https://github.com/ArchiveBox/ArchiveBox#%EF%B8%8F-easy-setup"><code>curl -sSL 'https://get.archivebox.io' | sh</code></a> (uses <code>pkg</code> + <code>pip3</code> under-the-hood)</li>
-<li>NixOS: <a href="https://github.com/NixOS/nixpkgs/issues/122793"><i>help wanted!</i></a></li>
+<li>Nix: <a href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/misc/archivebox/default.nix"><code>nix-env --install archivebox</code></a> (contributed by <a href="https://github.com/siraben"><code>@siraben</code></a>)</li>
 <li>More: <a href="https://github.com/ArchiveBox/ArchiveBox/issues/new"><i>contribute another distribution...!</i></a></li>
 </ul>
 See <a href="#%EF%B8%8F-cli-usage">below</a> for usage examples using the CLI, Web UI, or filesystem/SQL/Python to manage your archive.