Add: overview about .citation.cff files on GitHub

[lwasser]

This issue involves writing new text about this file in our guidebook. It is beginner friendly in that you could complete this totally on the github GUI or even in a hackmd document and we can help you with the PR if you aren't a github user!

Add information about the citation.cff file to the license page:

• [ ] how .cff files work - what they add to the repo
• [ ] how dates are tracked in this file
• [ ] how they integrate into / with zenodo