message: README: Add nix installation instructions
diff: @@ -126,6 +126,9 @@ Tarpaulin used to rely on Cargo as a dependency and then require an ssl install
  as well as other libraries but now it uses your system cargo simplifying the
  installation and massively reducing the install time on CI.
  +When using the [Nix](https://nixos.org/nix) package manager, the `nixpkgs.cargo-tarpaulin` package can be used.
  +This ensures that tarpaulin will be built with the same rust version as the rest of your packages.
  +
  ### Command line
  To get detailed help on available arguments when running tarpaulin call:
  