docs: update readme

Co-Authored-By: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

README.md

@@ -1,7 +1,23 @@
-# Lunatic Model
-[![Build Status](https://travis-ci.org/InseeFr/Lunatic-Model.svg?branch=master)](https://travis-ci.org/InseeFr/Lunatic-Model)
+# Java classes for the Lunatic Model
 
+Java classes for the Lunatic questionnaire model.
 
-Classes and converters for the Lunatic questionnaire model.
+This library offers serialization and deserialization for Lunatic json questionnaires.
 
-The documentation can be found in the [docs](https://github.com/InseeFr/Lunatic-Model/tree/master/docs) folder and browsed [online](https://inseefr.github.io/Lunatic-Model).
+## Implementation
+
+The model is described by the Java classes. [Jackson](https://github.com/FasterXML/jackson) is used for 
+serialization/deserialization.
+
+## The Lunatic Model
+
+Note that this project does not define the Lunatic model (although its name suggests it): 
+[Lunatic](https://github.com/InseeFr/) does (changes in the questionnaire model in Lunatic are brought in 
+Lunatic-Model in a second stage).
+
+In Lunatic, classes are defined in the 
+[type source typescript file](https://github.com/InseeFr/Lunatic/blob/3.0/src/type.source.ts).
+
+## Documentation
+
+Documentation can be found in the [docs](https://github.com/InseeFr/Lunatic-Model/tree/master/docs) folder and browsed [online](https://inseefr.github.io/Lunatic-Model). (Needs to be updated.)