README: split up into docs

docs/nixos.md

@@ -0,0 +1,16 @@
+# Use with NixOS
+
+For NixOS, there is a module available via the nix flake. You can include it in your configuration like this:
+
+```nix
+{
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-XX.YY";
+
+  inputs.muscl.url = "git+https://git.pvv.ntnu.no/Projects/muscle.git";
+  inputs.muscl.inputs.nixpkgs.follows = "nixpkgs";
+
+  ...
+}
+```
+
+The module allows for easy setup on a local machine by enabling `services.muscl.createLocalDatabaseUser`.