readme: corrections and clarifications
readme: corrections and clarifications

Make it clear, that JSO is not an indepentend ASS-implementation; so
there are less compatability concerns.

libass doesn't actually support _all_ SSA (AlphaLevel),
ASS (fadeaway{width,height}, WrapStyle) and v4++ (\kt) features.

JSO doesn't actually support eg Graphite fonts, due to not including
libgraphite in its HarfBuzz builds.

Dador/JavascriptSubtitlesOctopus,Dador/JavascriptSubtitlesOctopus