Add build instructions for Nix

README.adoc

@@ -43,7 +43,7 @@ image:https://zenodo.org/badge/DOI/10.5281/zenodo.7892181.svg?style=social&label
 http://www.nic.funet.fi/pub/sci/physics/elmer/bin/windows[Windows Installers],
 http://www.nic.funet.fi/pub/sci/physics/elmer/bin/VirtualMachines/[VirtualBox VM]
 * Ubuntu Launchpad: https://launchpad.net/~elmer-csc-ubuntu/+archive/ubuntu/elmer-csc-ppa[PPA]
-* Compilation:  https://github.com/ElmerCSC/elmerfem/blob/devel/compilation_instructions/Ubuntu.md[Linux/Ubuntu],  https://github.com/ElmerCSC/elmerfem/blob/devel/compilation_instructions/Windows-msys2.md[Windows/msys2], https://github.com/ElmerCSC/elmerfem/blob/devel/compilation_instructions/macOS.md[macOS]
+* Compilation:  https://github.com/ElmerCSC/elmerfem/blob/devel/compilation_instructions/Ubuntu.md[Linux/Ubuntu], https://github.com/ElmerCSC/elmerfem/blob/devel/compilation_instructions/nix.md[Nix], https://github.com/ElmerCSC/elmerfem/blob/devel/compilation_instructions/Windows-msys2.md[Windows/msys2], https://github.com/ElmerCSC/elmerfem/blob/devel/compilation_instructions/macOS.md[macOS]
 * Docker: https://github.com/ElmerCSC/elmerfem/blob/devel/docker/elmer.dockerfile[GitHub], nwrichmond/elmerice: https://hub.docker.com/r/nwrichmond/elmerice/[Docker Hub]
 // * unifem/Elmer-desktop: https://github.com/unifem/Elmer-desktop[GitHub]
 // * CoSci-LLC/docker-elmerice: https://hub.docker.com/repository/docker/coscillc/elmerice[Docker Hub], https://github.com/CoSci-LLC/docker-elmerice[GitHub]

compilation_instructions/nix.md

@@ -0,0 +1,27 @@
+# Compilation of Elmer under Nix
+
+## Compiling from upstream
+
+The Nix flake exposes three different derivations (packages) that can be built using `nix build github:ElmerCSC/elmerfem#<version>`.
+
+### `default`
+Barebones Elmer with OpenMP and MPI support.
+
+### `gui`
+Elmer with GUI.
+
+### `ice`
+Elmer/ICE
+
+Specifying no version builds the `default` derivation.
+
+After building, the binaries are accessible under `result/bin`.
+
+## Compiling from a local repository
+
+In the local Elmer repository run `nix build` or `nix build .#<version>`.
+
+## Verbose output
+
+By default `nix build` only shows one line of the build output at once.
+To enable more verbose output, add the `-L` flag to the end of the build command.