Setup complete documentation

[ravage84]

Especially if this tools becomes bigger and gets more features, it would benefit from having a complete documentation.
This documentation should not live in the README.md, as it would bloat it over time.

Such a documentation could live in an informational website (#17), on an external host such as Read the Docs or simply as a collection of linked Markdown files in the docs folder or a mixture of them (like Composer).

Such a documentation should also document how to extend the current library, if necessary or possible.