Contributing guide enhancement
This merge request fix the issue #824 (closed).
In addition to describing the actions needed to initialize the project in SonarQube, I have rewritten the beginning of the contributing guide to present the various ways to contribute to the project.
Finally, I updated the screenshots showing the fork and merge request buttons, and then updated the message displayed by the CI script as requested in the issue description.
Edited by Sébastien Dinot