nixos/zapret: mention that `blockcheck` requires `nftables` or `iptables`

        Specify the bypass parameters for Zapret binary.

        There are no universal parameters as they vary between different networks, so you'll have to find them yourself.

        This can be done by running the `blockcheck` binary from zapret package, i.e. `nix-shell -p zapret --command blockcheck`.

        This can be done by running the `blockcheck` binary from zapret package, i.e. `nix-shell -p nftables zapret --command blockcheck` (or `iptables` instead of `nftables` if that is what your firewall is using).

        It'll try different params and then tell you which params are working for your network.

      '';

    };