examples: add importable-configuration.nix

This replaces minimal-configuration.nix.
importable-configuration.nix can be directly added to an existing
NixOS configuration. This makes it easy for users to get started quickly.

examples/importable-configuration.nix

@@ -0,0 +1,38 @@
+# You can directly copy and import this file to use nix-bitcoin
+# in an existing NixOS configuration.
+# Make sure to check and edit all lines marked by 'FIXME:'
+
+# See ./flakes/flake.nix on how to include nix-bitcoin in a flake-based
+# system configuration.
+
+let
+  # FIXME:
+  # Overwrite `builtins.fetchTarball {}` with the output of
+  # command ../helper/fetch-release
+  nix-bitcoin = builtins.fetchTarball {};
+in
+{ config, pkgs, lib, ... }: {
+  imports = [
+    "${nix-bitcoin}/modules/modules.nix"
+  ];
+
+  # Automatically generate all secrets required by services.
+  # The secrets are stored in /etc/nix-bitcoin-secrets
+  nix-bitcoin.generateSecrets = true;
+
+  # Enable some services.
+  # See ./configuration.nix for all available features.
+  services.bitcoind.enable = true;
+  services.clightning.enable = true;
+
+  # Enable interactive access to nix-bitcoin features (like bitcoin-cli) for
+  # your system's main user
+  nix-bitcoin.operator = {
+    enable = true;
+    # FIXME: Set this to your system's main user
+    name = "main";
+  };
+
+  # Prevent garbage collection of the nix-bitcoin source
+  system.extraDependencies = [ nix-bitcoin ];
+}