Add a "Setup" section to the README

[kelson42]

Explaining how to get the tool ready to use.

This should cover: linux, macos and windows.

[rgaudin]

I don't get what you're expecting ; there's already a Usage section.

It's a tool for power users and it's unix only. It's docker-only ATM and we have #1 (we don't want the hassle of supporting end-users with the dev stack).

[kelson42]

I just want to have the step between a raw linux and running the command itself ("usage" section).

If docker can run on macOS or on Windows WSL, then it should be clear how to do it.

Let us just try to lower a bit the first step.