docs(readme): Document pre-flake usage

README.md

@@ -115,7 +115,41 @@ $ dub --version
 DUB version 1.30.0, built on Jan  1 1980
 ```
 
-See complete example in [`templates/devshell/flake.nix`](./templates/devshell/flake.nix).
+You can find the full example in [`templates/devshell/`](./templates/devshell/).
+
+### Pre-flakes usage
+
+The `default.nix` file in the root of this repo exposes all flake outputs as
+Nix attributes. This is useful for users who haven't yet made the jump to the
+Nix Flakes world.
+
+For example, if you have an existing `shell.nix` file, all you need to do is add
+the changes marked as "NEW" from the snippet below:
+
+```nix
+{pkgs ? import <nixpkgs> {}}: let
+  # NEW: Import the dlang-nix Nix library:
+  dlang-nix = import (pkgs.fetchFromGitHub {
+    owner = "PetarKirov";
+    repo = "dlang.nix";
+    rev = "3502a9f6dd2074c2f84d49baa5043f6601ca6407";
+    hash = "sha256-djp8c2iONh+ET+wHbPLruNTuF7xSAYoMmwp1HfsrVTA=";
+  });
+
+  # NEW: Add `dpkgs` shorthand:
+  dpkgs = dlang-nix.packages."${pkgs.system}";
+in
+  pkgs.mkShell {
+    packages = [
+      # NEW: Reference D-related packages from `dpkgs`:
+      dpkgs.dmd
+      dpkgs.dub
+    ];
+  }
+```
+
+You can find the full example in
+[`templates/pre-flake-devshell/`](./templates/pre-flake-devshell/).
 
 ## Source and binary variants
 

templates/pre-flake-devshell/.envrc

@@ -0,0 +1,8 @@
+# shellcheck shell=bash
+if ! has nix_direnv_version || ! nix_direnv_version 3.0.4; then
+  source_url "https://raw.githubusercontent.com/nix-community/nix-direnv/3.0.4/direnvrc" "sha256-DzlYZ33mWF/Gs8DDeyjr8mnVmQGx7ASYqA5WlxwvBG4="
+fi
+
+dotenv_if_exists
+watch_file shell.nix
+use nix

templates/pre-flake-devshell/shell.nix

@@ -0,0 +1,21 @@
+{pkgs ? import <nixpkgs> {}}: let
+  dlang-nix = import (pkgs.fetchFromGitHub {
+    owner = "PetarKirov";
+    repo = "dlang.nix";
+    rev = "3502a9f6dd2074c2f84d49baa5043f6601ca6407";
+    hash = "sha256-djp8c2iONh+ET+wHbPLruNTuF7xSAYoMmwp1HfsrVTA=";
+  });
+
+  dpkgs = dlang-nix.packages."${pkgs.system}";
+in
+  pkgs.mkShell {
+    packages = [
+      pkgs.figlet
+      dpkgs.dmd
+      dpkgs.dub
+    ];
+
+    shellHook = ''
+      figlet "Hello, D world!"
+    '';
+  }