update README to recommend nix users install with flake.nix

I added a flake to Termonad in https://github.com/cdepillabout/termonad/pull/202 for use by Nix users.

After flakes are fully released and no longer experimental, I should update the README to explain how to install from the `flake.nix` instead of the normal `nix-build default.nix` method.