Add limitations to README (accessibility)

[missmatsuko]

I think you should add a warning to the README that the CSS version of this font should not be used in production and to use the OTF/TTF version instead.

From the examples, it doesn't seem like there is any way to make the font screenreader-friendly.

Additionally, it seems that some of the characters use a white background colour to to "mask" parts of letters, which means it's be difficult to use the CSS on anything other than a white background (or else you'd have to edit the colours in a bunch of places).