add cache to example

Author: tstat

README.md

@@ -26,6 +26,12 @@ complete example.
 ```nix
 {
   description = "raspberry-pi-nix example";
+  nixConfig = {
+    extra-substituters = [ "https://raspberry-pi-nix.cachix.org" ];
+    extra-trusted-public-keys = [
+      "raspberry-pi-nix.cachix.org-1:WmV2rdSangxW0rZjY/tBvBDSaNFQ3DyEQsVw8EvHn9o="
+    ];
+  };
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-22.11";
     raspberry-pi-nix.url = "github:tstat/raspberry-pi-nix";