Add a README that references somehow the key primitives this presents for the UI

Let's flesh out the README a bit better by mentioning at least the more key query (get trace by ID) in a summary section. Please use curl or httpie for that.

Below that should describe the primary queries used for the main screens. If there's a tool to generate this like swagger has, that's fine also, but it should be easier for people to know what this project is serving.