Update CONTRIBUTING.md

[kerautret]

PR Description

Updating Contributing from previous CLI11 changes.

Checklist

• [ ] Doxygen documentation of the code completed (classes, methods, types, members...)
• [ ] Check if it follows the tools structure described in CONTRIBUTING.md
• [ ] New entry in the ChangeLog.md added.
• [ ] Update the readme with potentially a screenshot of the tools if it applies.
• [ ] No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).