modelbuilder: document group in the base directory (the one before src)

Search also `README.md` that is already used in some programs in contrib.

Signed-off-by: Jean Privat <jean@pryen.org>
Merge: Search `README.md` to document groups

Pull-Request: #501
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>