Document specifying secrets for CHAP authentication

[Patricol]

To save the next person some time - as best I can tell from the code - you add a volumeAttribute (to the pv) named 'secret'; with a value matching this pseudocode format:

jsonencode({
  "node.session.auth.username": "your userid"
  "node.session.auth.password": "your password"
  "node.session.auth.username_in": "your mutual_userid"
  "node.session.auth.password_in": "your mutual_password"
  "node.sendtargets.auth.username": "your discovery userid"
  "node.sendtargets.auth.password": "your discovery password"
  "node.sendtargets.auth.username_in": "your discovery mutual_userid"
  "node.sendtargets.auth.password_in": "your discovery mutual_password"
})