add nix build instructions

.gitignore

···
 
 # ttf converter binary
 BitsNPicas.jar
+.envrc
+result

default.nix

···
+let 
+  pkgs = import <nixpkgs> {};
+in
+  with pkgs;
+  stdenv.mkDerivation rec {
+    name = "scientifica";
+    version = "v0.1.0";
+    src = ./src;
+    buildInputs = [ fontforge python3 xorg.mkfontdir ];
+
+    buildPhase = ''
+      for i in ./*; do
+          fontforge -c 'open(argv[1]).generate(argv[2])' $i $i.otb
+      done
+      '';
+    installPhase = ''
+      fontDir="$out/share/fonts/misc"
+      install -m 644 -D *.otb out/* -t "$fontDir"
+      mkfontdir "$fontDir"
+      '';
+  }
+
+

shell.nix

···
+{ pkgs ? import <nixpkgs> {} }:
+
+with pkgs;
+mkShell {
+  buildInputs = [ fontforge python3 xorg.mkfontdir ];
+}