improve / add JSON documentation

[DaveG43]

I love this plugin, and was wondering if you are still planning on developing the json feature. Currently I can't get it to work. Will there be documentation coming?

[robintindale]

The API probably won't be too stable for a while, as we are just using this internally for now. Biggest missing feature currently is that you can't customize the font face.

Otherwise - you can hopefully see how it's intended to work from the two token file examples

https://git.zeroheight.com/robin/tokens/raw/master/typography.json
https://git.zeroheight.com/robin/tokens/raw/master/colors.json