Get a DOI code for pele

[smcantab]

we should register pele on https://zenodo.org/ to get a doi. We can then add the label to the github README and users can cite it consistently

[smcantab]

this requires making a release, any objection? We could do one every 6 months or so or just do it once, just so that we have a DOI to point to (although strictly speaking every time that a new functionality is added and used the release should be updated or the DOI points to the wrong version of the code)