Update README to show all OpenType features

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](https://github.com/microsoft/cascadia-code#font-features), [JetBrains Mono](https://github.com/JetBrains/JetBrainsMono/blob/master/README.md#opentype-features), or the old one like [Jira Code](https://github.com/tonsky/FiraCode/blob/master/README.md), 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](https://github.com/adobe-fonts/source-code-pro/blob/main/Upright/familyGSUB.fea#L370-L676). 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.