Create a CONTRIBUTING.md
Create a CONTRIBUTING.md

This file is displayed when contributors start a pull request.

It can be improved in the future with contribution guidelines specific for ADL.
Fabien-Chouteau/Ada_Drivers_Library,Fabien-Chouteau/Ada_Drivers_Library,simonjwright/Ada_Drivers_Library,simonjwright/Ada_Drivers_Library,AdaCore/Ada_Drivers_Library,AdaCore/Ada_Drivers_Library,ellamosi/Ada_BMP_Library