Add a contributing guide

Summary

Provide a contributing guide for new comers (and experienced developers) which explains how we work and what are the best practices.

What is the current behavior?

The CONTRIBUTING.md file is currently empty.

What is the expected correct behavior?

The CONTRIBUTING.md file should contain relevant information for all developers, such as :

• The forking model, i.e. non-forking
• The formatting style, i.e. WebKit
• The CERN email address enforcement
• The enforced branches names
• How to write a good Git commit message with a link to a good reference
• The link to the C++ Code Guidelines
• ...

(The README will have to be adapted in consequence.)