[Doc] Add a contribution guide

[sylvlecl]

• Do you want to request a feature or report a bug?

Doc

• What is the motivation ?

We need to document how people can contribute, in particular contribute to the documentation. See for reference the guide of pandas project, in particular guidelines for docstrings which is very well written, and which we could refer to.

That would consist in adding a top section "Contributing", at the same level as "Getting Started" etc.