Mark fields that are used by shields.io #1001
Description
The shields.io service expects certain fields to exist in the JSON response of 'https://crates.io/api/v1/crates/' + crate -- listed here. In badges/shields#1059 it looks like their license badge either never worked (?) or was broken at some point by a crates.io change.
Let's put a comment above fields that are used by shields.io so that if they need to change in the future, we know to follow up with a PR to shields.io.