Add documentation on exporting woff2 variable files

Hey @simoncozens , thanks for fixing #350 !

I can't figure out how to make use of your fix though 😬. Is there something I can change in the `stat` key in my config.yaml file?

I've been searching the documentation here and don't see anything about variable WOFF2: https://github.com/googlefonts/gftools/tree/main/docs/gftools-builder

(For reference, this is my config.yaml file: https://github.com/erikdkennedy/figtree/blob/master/sources/config.yaml)

Thanks 🙂