Nix configuration directory for the MMGen Node Tools suite

Usage is as described in ‘nix/README’ in the mmgen-wallet repository, with the
following differences:

  a) all commands are executed from the repository root of mmgen-node-tools
     instead of mmgen-wallet

  b) for NixOS, complete the steps as described up until the rebuild step.
     Copy the contents of this directory to ‘/etc/nixos/mmgen-project’ (this
     will overwrite ‘default.nix’), and continue with the rebuild step.