Add open-source licence and citable DOI

With @maudelejeune, we decided to put the permissive open-source BSD 3-clause licence on this project.

We should

• Add the licence file at the root of the project
• Add the licence file in the documentation
• Add the licence header at the top of each file
• Add a "Use policy" section in the README
• Create a Zenodo entry for LISA Constants (should we use version-specific DOI?)
• Add Zenodo DOI and pipeline badges in the README