added flake for development

flake.nix

@@ -0,0 +1,35 @@
+{
+  description = "Asist Development Flake";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    systems.url = "github:nix-systems/default";
+  };
+
+  outputs =
+    { nixpkgs, systems, ... }:
+    let
+      eachSystem = f: nixpkgs.lib.genAttrs (import systems) (system: f nixpkgs.legacyPackages.${system});
+    in
+    {
+      devShells = eachSystem (pkgs: {
+        default = pkgs.mkShell {
+          name = "asist";
+          buildInputs = with pkgs; [
+            gcc
+            openssl.dev
+            pkg-config
+            libiconv
+            rustc
+            cargo
+            (ruby.withPackages (
+              p: with p; [
+                asciidoctor
+                bundler
+              ]
+            ))
+          ];
+        };
+      });
+    };
+}