Document how to override attributes in configuration.nix

configuration.nix

@@ -2,7 +2,7 @@
 # your system.  Help is available in the configuration.nix(5) man page
 # and in the NixOS manual (accessible by running ‘nixos-help’).
 
-{ config, pkgs, ... }: {
+{ config, pkgs, lib, ... }: {
   imports = [
     ./modules/nix-bitcoin.nix
 
@@ -22,6 +22,17 @@
   # you are doing.
   services.nix-bitcoin.enable = true;
 
+  ### BITCOIND
+  # Bitcoind is enabled by default if nix-bitcoin is enabled
+  #
+  # You can override default settings from nix-bitcoin.nix as follows
+  # services.bitcoind.prune = lib.mkForce 100000;
+  #
+  # You can add options that are not defined in modules/bitcoind.nix as follows
+  # services.bitcoind.extraConfig = ''
+  #   maxorphantx=110
+  # '';
+
   ### CLIGHTNING
   # Enable this module to use clightning, a Lightning Network implementation
   # in C.