Add document for supported Characters/Languages in README

[xring]

As mentioned in #8, seems like only Latin characters are supported currently.

It will be great to add document to track supported / WIP / TODO Characters/Languages, it can also help community to work on related task together.

[robertknight]

Done! - Regarding collaboration around more languages, I first need to add the basic infrastructure (eg. better model training documentation), and then it will be possible for people with expertise in various languages to contribute. #8 is the main issue tracking this.