Add 'credential'

[yongqunh]

As Alabama government focuses on credential in their occupatio related research, we need to ontologically represent 'credential' term in OccO.

Barry et al has recently published a paper titled "On Credentials":
https://philpapers.org/archive/SMIOC-2.pdf

We would also like to like our ontology modeling to the handbook "Credential Engine Schemas Handbook"
https://credreg.net/ctdl/handbook
where they call their system "Credential Transparency Description Language (CTDL)"
Note that they did not use any upper level ontology like BFO.

[yongqunh]

See what is described in our OccO paper:
"Following [34], credential can be understood as a document that reflects an attestation to the
holder’s competence with respect to a given task, at or above a certain threshold. Accordingly,
credentials are issued by some authority and are numerous."
The reference: https://philpapers.org/archive/SMIOC-2.pdf