added default.nix for nix dev env

.nix/default.nix

@@ -0,0 +1,20 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+let
+  rustPkgs = with pkgs; [
+    rustc
+    cargo
+    rustfmt
+    rust-analyzer
+    clippy
+  ];  
+  pkgDeps = with pkgs; [
+    openssl
+    pkgconfig
+    sqlite
+  ];
+in
+  pkgs.mkShell {
+    nativeBuildInputs = rustPkgs ++ pkgDeps; 
+    RUST_BACKTRACE = 1;
+  }