Merge branch '23-add-specific-contributing-guide' into 'master'

Resolve "Add Specific contributing guide"

Closes #23

See merge request !20

CONTRIBUTING.md

+# Contributing
+
+Dev contributing guide.
+
+## General instructions
+
+Please read the [Concordant general contributing instructions](https://gitlab.inria.fr/concordant/internal/documentation/-/blob/master/CONTRIBUTING.md)
+
+## Branches
+
+Contributions are organised in a dev phases release cycle.
+
+The `master` branch is the latest stable release, tagged with release number.
+
+Wanted features, bug fixes, ideas are stagged as Issues, and developed as sprints.
+
+The `dev` branch is always open (so, never closed) for current development, all accepted merge requests are merged in this dev branch.
+
+At release time, the `dev` branch is simply merged into master, which means new deployement, and assigned a release version number depending on the features and cycle weight.
+
+## Version tags
+
+Each merge to master is a new prod deployement that needs to be tagged as a new version.
+
+We use a `x.y.z` release number model, where:
+
+- x: refers to major update;
+- y: new features update;
+- z: bug fix, minor change update.