Three tiered documentation

[vil1]

Before releasing a first version, we must provide a three-tiered documentation, each tier targeting a specific "level" of interaction with the library.

• User : the general philosophy behind the library, its intended purpose and goals; step-by-step descriptions of the most common use-cases (typeclass derivation, schema evolution), for every provided module.
• Power-user : how to extend the library, how the derivation mechanism works internally and how to create new modules and interpreters, how the migration mechanism works and how to implement new/custom migrations.
• Contributor : the design decisions that have lead to the current implementation and the reasons behind them, and more generally anything that is useful to know before contributing to the core of the library.