Add a cite field

[kofalt]

Based on an out-of-band request to display citation information of a gear.

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Is this a semantic or operational change? If so:

• [x] Increment the version in spec/readme.md
• [ ] After merge, tag the version and update the release page

[lmperry]

LGTM: Single string is good. Could be multiple citations, but line sep or other will be fine.