• Renamed "Firebase SDK" to "Firebase Admin SDK for PHP" to emphasize the similarity to the newly
  introduced official Admin SDKs.
• Added method Reference::getPath() to retrieve the full relative path to a node.
• Updated docs to make clearer that authenticating with a Database Secret is not recommended since
  the official deprecation by Firebase (see
  the "Database Secrets" section in the "Service Accounts" tab of a project
  )
• It is now possible to pass a JSON string as the Service Account parameter on Firebase::fromServiceAccount(). Until now, a string would have been treated as the path to a JSON file.