Explain file path for Secrets (#1316)
Explain file path for Secrets (#1316)

* Suzanne's edits

* Add use cases for file based secrets

mesosphere/dcos-commons,vishnu2kmohan/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons,vishnu2kmohan/dcos-commons,vishnu2kmohan/dcos-commons,vishnu2kmohan/dcos-commons,vishnu2kmohan/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons