Describe the different font versions

[acrostich]

It would be helpful for those with little knowledge of fonts to have a short description that would help choose which one to install.

For instance on macOS I went with the variable one as that intuitively seems more versatile. I initially tried to installed both variable ttf to realise that they conflict so went with the HEXP(*) as it's a bigger file and hence may support things that the other one doesn't. I may have lucked out on the best choice for my situation... or not.

Explaining some of the differences between the files that would help choose the right one for most situations non-experts would encounter would make this repository better.

(*) I eventually found that HEXP expands to hyper expansion (mentioned in google/fonts#3203 (comment))