doc: apply license and add contribution guide

[cmdoret]

• Apply GPL license headers
• Add contributing guide
• Add development section

[caviri]

Hi @cmdoret I think it covers the basics for contributing guidelines. In the future we can add more instructions about which kind of tests should pass one branch before sending us the PR.

Also, another detail in the license:

// Copyright (C) 2023 - Swiss Data Science Center (SDSC)
// A partnership between École Polytechnique Fédérale de Lausanne (EPFL) and
// Eidgenössische Technische Hochschule Zürich (ETHZ).

Shouldn't the PSI appear here too? I'm not sure about this .

[cmdoret]

Thanks! Indeed, this information would be important! I"ll add it here so we have an example

For the license header, the EPFL page of SDSC PSI is indeed mentioned, but it still says "The Swiss Data Science Center (SDSC) is a joint venture between EPFL and ETH Zurich.". I guess we could keep it as-is at least for now.