Add Nix flake

flake.nix

@@ -0,0 +1,26 @@
+{
+  description = "Website for all things related plain text accounting";
+  nixConfig.bash-prompt = "plaintextaccounting.org$ ";
+
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+    let pkgs = nixpkgs.legacyPackages.${system}; in {
+      packages = rec {
+        default = pkgs.stdenvNoCC.mkDerivation {
+          pname = "plaintextaccounting.org";
+          version = "master-${self.shortRev or "dirty"}";
+          src = self;
+          nativeBuildInputs = with pkgs; [pandoc];
+          installPhase = ''
+            runHook preInstall
+            mkdir -p $out/quickref
+            cp -R index.html css images $out
+            cp quickref/*.html $out/quickref
+            runHook postInstall
+            '';
+        };
+      };
+  });
+}