add Readme file

Signed-off-by: Camille Tardy <camille.tardy@unige.ch>

README.md

+## University of Geneva Boost child theme
+
+Boost Child theme for Moodle 3.5.
+A related Mahara theme can be found [in our Gitlab](https://gitlab.unige.ch/tardyc/Theme-Mahara)
+
+
+*Author : University of Geneva, 2018*
+