Better documentation required

[jaykru]

The existing API documentation seems to be simply auto-generated from the .mli files. It's not clear to me after a few hours of poring over the .mlis and even source files how to accomplish basic functionality like appending a character to the end of the text. A short overview à la https://pqwy.github.io/notty/doc/Notty.html with a couple of simple examples would go a long way.