Add citation details

[m-mohr]

Related Issue(s): None

Proposed Changes:

Add a citation file. The main issue is that it only allows software or dataset, not techreport or specification, but we are following their advice here: https://github.com/citation-file-format/citation-file-format/issues/403

PR Checklist:

• [ ] This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• [x] This PR has no breaking changes.
• [x] I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[m-mohr]

@cholmes @matthewhanson @lossyrob This needs to additional reviews.