[PATCH 2 of 2] formatter: add overview of API and example as doctest

[Yuya Nishihara]

On Sun, 27 Nov 2016 14:26:03 +0000, Kostia Balytskyi wrote:
> I like this and previous patch, but if the purpose of this one is to provide
> examlpes to developers, should we maybe also include debugformatter?

Good point, but debugformatter uses repr() so the output order is unstable.
I think jsonformatter serves well for debugging output.