Document the various data APIs

[oeed]

While the data is public isn't not at all obvious at the moment how to actually use the data. It'd be good to create a doc that illustrates how the data is structured and what's needed to use it.

[olafwrieden]

@oeed what can we do to move this issue forward? Can we please add transparency into how often we scrape which types of APIs as well as where our data comes from and how it is sourced? @wseagar and @MattOfNZ could you put an API readme together to document this?