Documentation how to use overlays

[teutat3s]

Thank you for these useful templates!

I'd like to use a nixpkgs overlay with this (use a different version of nix). Do you have an example how to use overlays with these templates? In my case I use mkARMMacosSystem.

I'd like to apply the following overlay to nixpkgs:

final: prev: {
  nix = final.nixVersions.nix_2_16;
}