Add CITATION.cff file and update README with full citation

[ml-evs]

Closes #97

Adds the "cite this repository" button on GitHub, with copyable citation that looks like:

Eimre, K., Evans, M. L., Macaulay, B., Wang, X., Yu, J., Marzari, N., Rignanese, G., & Pizzi, G. (2026). optimade-maker: automated generation of interoperable materials APIs from static datasets. Digital Discovery, 5. https://doi.org/https://doi.org/10.1039/D6DD00125D

or

@article{Eimre_optimade-maker_automated_generation_2026,
author = {Eimre, Kristjan and Evans, Matthew L. and Macaulay, Bud and Wang, Xing and Yu, Jusong and Marzari, Nicola and Rignanese, Gian-Marco and Pizzi, Giovanni},
doi = {https://doi.org/10.1039/D6DD00125D},
journal = {Digital Discovery},
title = {{optimade-maker: automated generation of interoperable materials APIs from static datasets}},
volume = {5},
year = {2026}
}

Will also automatically update Zenodo metadata when we next do a release

[eimrek]

Looks good! thanks! :)