document PO Token Provider URL

[bmcdonough]

This is a companion pull request to tubearchivist/tubearchivist#1076

If that request is merged, this should also be merged to document the PO Token Provider URL.

[bmcdonough]

@bbilly1
Would you provider guidance on if I should update the schema.yaml as well?
That file is a recent additional to this repo, and your previous documentation hinted that schema might become dynamically created. I do not want to clobber any dynamic creation of the schema if that is done elsewhere.

[bbilly1]

you can ignore schema.yaml, that updates during the release, almost automatically now.