Added

Many documentation improvements:

• add a Code of conduct
• add a Contributing guide -
• Refactor README to separate clearly from documentation
• Fix broken links
• Fix bad markdwon rendering
• Split old readme information in dedicated sections

Changed

• Enable pipeline_ml to accept artifacts (encoder, binarizer...) to be "intermediary" outputs of the pipeline and not only "terminal" outputs (i.e. node outputs which are not re-used as another node input). This closes a bug discovered in a more general discussion in #16.
• Only non-empty CLI arguments and options are logged as tags in MLflow (#32)