Add a citation file

[jonseddon]

✨ Feature Request

A citation file (CITATION.cff in the root of the repository) would contain information on how to cite Iris; GitHub would add a box explaining how to cite Iris and there is integration with Zenodo too. There's more information in https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files. Much of the information to create the file is in the Iris user guide at https://scitools-iris.readthedocs.io/en/latest/userguide/citation.html.

Motivation

This is a quick win, with minimal effort and may help to gather evidence of the use of Iris.

Additional context

This was first mentioned in PR #5116.