Clarify we can enable slashed zero by "zero" not only "ss19" in README

[tats-u]

Description of the new feature/enhancement (with images if possible)

There is the unified OpenType feature to enable the slashed zero: "zero".

It is adopted by JetBrains Mono and Inter. We should avoid the unstandardized way in the font installed in the standard programming font in a OS).

If we use it, we can enable it by font-variant-numeric: slashed-zero without font-feature-settings in CSS.

Proposed technical implementation details (optional)

[aaronbell]

Cascadia supports both zero and ss19, in cases where zero doesn't work.

[tats-u]

@aaronbell
Ah, you're right.

https://codepen.io/tats-u/pen/bGPxwNL

Then, could you add "zero" at the top or bottom of "ss19"?

[tats-u]

@aaronbell
Now I changed the title, could you reopen this issue?

[aaronbell]

Yes. I will be doing an overhaul of the OT documentation in time.

[tats-u]

Thank you for your super quick response.