Add instructions on how to proceed in README

[tcompa]

How do I install/use this application?
How do I run tests?

I'm looking at dev (should I look at a different branch?). I see a poetry.lock file, and then I try to poetry install. And indeed it does something, like installing a lot of dependencies and then also exact_rag.

And then? It'd be great to have an example (even just in README) or a working test.