Create a README file for the inclusion case

test_data/inclusion/README.md

+# Folder instructions
+
+This directory contains test data for models made up by a sphere with inclusions.
+
+# License
+
+   Copyright (C) 2024   INAF - Osservatorio Astronomico di Cagliari
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+   
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+   
+   A copy of the GNU General Public License is distributed along with
+   this program in the COPYING file. If not, see: <https://www.gnu.org/licenses/>.