message: Document new dot-separated notation
diff: @@ -24,6 +24,9 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  generalizing the function to `config_value` as well.
  - The `typed_response_promise` received additional member functions to mirror
  the interface of the untyped `response_promise`.
  +- Configuration files now allow dot-separated notation for keys. For example,
  + users may write `caf.scheduler.max-threads = 4` instead of the nested form
  + `caf { scheduler { max-threads = 4 } }`.
  ### Deprecated
  