Add contributing guidelines

[rmarquis]

In line with the refactoring effort (#443) and because GitHub encourages to implement proper open source guidelines: add contributing guidelines.

Ponctual contributors should be encouraged to participate, which will also favorise the emergence of more regular contributors.

Writing your contributing guidelines

A CONTRIBUTING file tells your audience how to participate in your project. For example, you might include information on:

How to file a bug report (try using issue and pull request templates)
How to suggest a new feature
How to set up your environment and run tests
In addition to technical details, a CONTRIBUTING file is an opportunity to communicate your expectations for contributions, such as:

The types of contributions you’re looking for
Your roadmap or vision for the project
How contributors should (or should not) get in touch with you
Using a warm, friendly tone and offering specific suggestions for contributions (such as writing documentation, or making a website) can go a long way in making newcomers feel welcomed and excited to participate.

This is already partially done in various place (translations repo, wiki), but should be consolidated and completed.

[ismaelgv]

I think that code restructuring and clean up will help a lot here. It is a good idea to summarize contribution guide in a file. Also, it would be interesting to add labels to some issues to drive possible contributors effort apart from community label, for example, 'easy', 'refactor', etc.