Add CITATION.cff file about uncertainties

[newville]

Adding a Zenodo citation for this project would be nice. Certainly, @lebigot E. O. Lebigot would be the lead author of that. I would suggest even having E. O. Lebigot as the sole author -- the commit history is pretty clear.

[newville]

@wshanks @lebigot I enabled the Zenodo webhook so that, on the next release, a DOI will be created that can be used as the citation. Still slowly working on the docs, and hope to have more time for this next week.

[wshanks]

Note how the original author asked for the package to be cited. I am not sure if or how this should ever change since he has handed over maintenance to the lmfit team:

uncertainties/doc/index.rst

Lines 385 to 392 in e9b82f0

	How to cite this package
	========================
	If you use this package for a publication (in a journal, on the web,
	etc.), please cite it by including as much information as possible
	from the following: *Uncertainties: a Python package for calculations
	with uncertainties*, Eric O. LEBIGOT. Adding the version
	number is optional.

[lebigot]

Honestly, I'm not against having all the contributors listed, but I'm sensitive to the commiters argument: a single author would represent the commit history (and the creation effort) more clearly indeed. Thanks!

[wshanks]

Indeed, I see no reason to change the citation at this time. What I was not sure about was how we would agree on changing it in the future if there are further contributions, but that is a question for a later date 🙂

[lebigot]

Thanks!!