adding flake with sbcl and direnv

flake.nix

@@ -0,0 +1,16 @@
+{
+  description = "learning Common Lisp";
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem
+      (system:
+        let pkgs = nixpkgs.legacyPackages.${system}; in
+        {
+          devShells.default = pkgs.mkShell {
+            buildInputs = [ pkgs.sbcl ];
+          };
+        }
+      );
+  # see https://serokell.io/blog/practical-nix-flakes
+
+}