Add NixOS to README.md

README.md

@@ -52,6 +52,7 @@ Full documentation is at netboot.xyz:
 * [Manjaro Linux](https://manjaro.github.io)
 * [Microsoft Windows](https://www.microsoft.com)
 * [MirOS](https://www.mirbsd.org)
+* [NixOS](https://nixos.org)
 * [OpenBSD](http://openbsd.org)
 * [OpenSUSE](http://opensuse.org)
 * [RancherOS](http://rancher.com/rancher-os/)