Clarify intended usage

The README seems to suggest that adding `cuda-maintainers.cachix.org` as a substituter and setting `allowUnfree = true` and `cudaSupport = true` is sufficient to get the prebuilt packages. However, quite often I end up rebuild some of the CUDA-enabled packages after updating. I have a few questions: 1) I currently have `nixpkgs` following `github:nixos/nixpkgs/nixos-unstable` in my flake and I run `nix flake update nixpkgs` every once in a while, but this seems like a bad strategy, because the CI might be lagging behind upstream and not every commit may be successfully built. Is there some better way to only track the `nixos-unstable` commits that were succesfully built by `nixpkgs-cuda-ci`? The README links to the [hercules dashboard](https://hercules-ci.com/github/SomeoneSerge/nixpkgs-cuda-ci), but it's not clear how to get the desired information from that dashboard. It also looks like most jobs are failing for some reason. 2) The README mentions that > We build for different cuda architectures at a different frequencies, which means that to make use of the cache you might need to import nixpkgs as e.g. import <nixpkgs> { ...; config.cudaCapabilities = [ "8.6" ]; }. Cf. the flake for details What are those "different frequencies" exactly? 3) [`nix/overlays.nix`](https://github.com/SomeoneSerge/nixpkgs-cuda-ci/blob/5c27f5ae8002c15ec092a6ef93cc00d86b627408/nix/overlays.nix#L60) seems to also be optionally enabling MKL versions of LAPACK/BLAS. Are these versions of packages also built in CI and if so, how often? So, for example, if I set `cudaCapabilities = [ "8.6" ]` and enable the MKL the same way as your `nix/overlay.nix`, how can I determine the latest `nixos-unstable` commit that is already available in `cuda-maintainers.cachix.org`?