Update README to show all OpenType features

[boonyasukd]

It's now quite a common place for programming fonts to offer OpenType's character variants (cvXX) or stylistic sets (ssXX).

More recently introduced fonts like Cascadia Code, JetBrains Mono, or the old one like Jira Code, have laid out all possible features that users can customize within their READMEs. For Source Code Pro though, I actually have to dig down into source code. And even then, I still don't know how text will look until I apply the features, since there's no visual examples to look at beforehand.

It'd be great if there's some visual examples to explain how each feature looks like within a README.

[pauldhunt]

I’ll look into this.