WIP: protocol : rework tree structure

keys/README.md

-# Introduction
+# keys
 
 `keys` is a crate managing cryptographic keys for the Duniter project.
 
 [Duniter]: https://duniter.org/en/
 
-# How to use it ?
+## How to use it
 
 You can add `duniter-keys` as a `cargo` dependency in your Rust project.