Document usage with GitHub Actions?

Thanks for miniforge!

The pending release of [setup-miniconda](https://github.com/marketplace/actions/setup-miniconda) (already works on `@master`) supports [custom installers](https://github.com/marketplace/actions/setup-miniconda#example-5-custom-installer), with the tested example being miniforge for pypy on linux and osx :tada:.

Might it be appropriate to link over to it?

It might also be worth exploring the generation of an `index.html` compatible with the [miniconda archive](https://repo.anaconda.com/miniconda/) such that it could be more-or-less a drop-in replacement for tools that know how to parse that format.