Add reference to the new community NixOS package

docs/getting-started.md

@@ -29,9 +29,10 @@ If you would like a high-level view of the infrastructure and how each feature i
 
 ## Install
 Install via:
+- [Docker image](install/docker.md)
 - [Debian package](install/debian.md)
 - [ArchLinux](install/archlinux.md)
-- [Docker image](install/docker.md)
+- [NixOS](install/nixos.md)
 - [Sources](build.md)
 
 See the [Latest release](https://github.com/kamax-matrix/mxisd/releases/latest) for links to each.

docs/install/nixos.md

@@ -0,0 +1,8 @@
+# NixOS package
+mxisd is available as a NixOS package in the official repos.
+
+It is maintained by [maximilian](https://matrix.to/#/@maximilian:transformierende-gesellschaft.org), a community member.  
+
+Related resources:
+- [NixOS](https://nixos.org/)
+- [The module definition](https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/networking/mxisd.nix)