Add new style citation file

[polm]

Description

This adds the new-style CITATION.cff file and places the older Bibtex formatted CITATION file.

Types of change

docs update

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[svlandeg]

This will need further updates later, but for now it's a good idea to use the new format at least :-)

[sethidden]

Hmm I can't convert the .cff file to the bibtex fromat using https://github.com/citation-file-format/cff-converter-python
Is this a valid file?

$ pip3 install cffconvert
$ cffconvert --url https://raw.githubusercontent.com/explosion/spaCy/master/CITATION.cff -f bibtex
'https://raw.githubusercontent.com/explosion/spaCy/master/CITATION.cff' does not pass validation. Conversion aborted.

Same if I clone the repo and run in on the file, or curl just the file etc. I'm on Linux but I doubt it's some CRLF issue tbh
cc @polm please let me know if this looks valid and I'll create an issue

[polm]

Github picks the file up just fine. Looking at the README for that project, is this the issue?

cffconvert does not support converting items from references or preferred-citation keys at the moment.

Our file uses preferred-citation because we refer to a paper and not the repo.