Upon submission, automatically encourage submitter to add missing fields

[adam-coster]

When someone creates a PR we have a GitHub workflow that runs the library-listing compiler to check for issues.

That workflow could also do a diff of the prior version of the data, so that it could see exactly what the submitter added. If any of the optional-but-really-useful fields are missing, the workflow could create a comment on the PR notifying the submitter that they should consider filling those fields out.

This would reduce the workload on reviewers.