Add CITATION.cff file, update README.md with DOI and version

[durack1]

I implemented a CITATION.cff file into the WCRP-CMIP/CMIP6_CVs a while back (WCRP-CMIP/CMIP6_CVs#1231), and it would be great to do the same here. The template looks something like

cff-version: 1.2.0
message: "If you use this software, please cite it using the metadata from this file"
authors:
- family-names: "Durack"
  given-names: "Paul J."
  orcid: "https://orcid.org/0000-0003-2835-1438"
- family-names: "Taylor"
  given-names: "Karl E."
  orcid: "https://orcid.org/0000-0002-6491-2135"
- family-names: "Mizielinski"
  given-names: "Matthew"
  orcid: "https://orcid.org/0000-0002-3457-4702"
- family-names: "Doutriaux"
  given-names: "Charles"
  orcid: "https://orcid.org/0009-0002-6932-3169"
- family-names: "Nadeau"
  given-names: "Denis"
  orcid: "https://orcid.org/0000-0002-9622-3289"
- family-names: "Juckes"
  given-names: "Martin"
  orcid: "https://orcid.org/0000-0003-1770-2132"
title: "CMIP6 Controlled Vocabularies (CVs)"
version: 6.2.58.68
doi: 10.5281/zenodo.12197150 
date-released: 2024-06-20
url: "https://github.com/WCRP-CMIP/CMIP6_CVs"

This will require updating with contributor information, pulling as much info from the github profile as possible.

While we're at it, it would also be great to add the Zenodo DOI, which will be autogenerated every time a new release is triggered. The repo is already connected to Zenodo, but we haven't triggered a release since the connection was made so are awaiting this for a DOI to be issued (a Paul/account only link is here which should become public once the release occurs).

We also need to update the version number in the README.md (and CITATION.cff) automagically. We'll need to think a little about this once we have the latest changes in main, I'm sure there'll be some tweaks, so not an urgent item.

Some code that updates info (not contributors/authors) automatically can be found in the CMIP6_CVs/src/writeJson.py