Add some hint on howto get valid signing keys.

nixos/modules/services/networking/nix-serve.nix

@@ -31,6 +31,15 @@ in
         default = null;
         description = ''
           The path to the file used for signing derivation data.
+          Generate with:
+
+          ```
+          nix-store --generate-binary-cache-key key-name secret-key-file public-key-file
+          ```
+
+          Make sure user `nix-serve` has read access to the private key file.
+
+          For more details see <citerefentry><refentrytitle>nix-store</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
         '';
       };