Send a welcome email to a new contributor

Once a new contributor is detected he should receive a welcome email containing :

• Thank for your interest in the project
• List of contacts that could help in case of difficulties
• A reminder of the role of the contributors
• A how to list of different things to do : how to install an IDE to fix the issue, how to launch, how to test, how to create the PR...
• Next steps to become a committer : CLA