updates to data node contacts file in this repo

[alaniwi]

As the data nodes contacts list is now public at docs/data-node-contacts.md (having been moved from a private site), we should:

• add something to advise end-users regarding the use of this list:

  • what sort of errors might indicate that it is/isn't a data node issue
  • if it appears data-node related, then how to find out which data node they are using as opposed to index node or openid provider
  • otherwise, the appropriate alternative channel to ask for help

• consider spam-proofing the email addresses somehow (maybe separate table columns for domain and local part of the address)

[alaniwi]

(Couldn't think of a more descriptive title for the ticket. Feel free to edit.)

[soay]

Thanks for pointing this out, Alan. I moved the page to the CDNOT secure repo.