Add info when init with cached Nixpkgs

[flandweber]

closes #384

[nmattia]

Thanks! Did you try it out and it works as expected?

[flandweber]

Did you try it out and it works as expected?

Yes, the formating seems to work:

ghci> :main init
Initializing
  Not updating sources.nix
  Not creating nix/sources.json
Done: Initializing

INFO: `niv init` didn't fetch the latest commit for nixpkgs (due to --fast).
      Run `niv update nixpkgs` if you wish to pin the latest.

[nmattia]

Thank you @flandweber !