update logger docs
# Common Logger
The *tornado_common_logger* crate contains the logger configuration for the Tornado components.
-The current implementation is based on [fern](https://github.com/daboross/fern).
The configuration is based on three entries:
-- __level__: Defines the logger verbosity. Valid values are: *trace*, *debug*, *info*, *warn*, and *error*.
+- __level__: A list of comma separated logger verbosity levels. Valid values for a level are: *trace*, *debug*, *info*, *warn*, and *error*.
+ If only one level is provided, this is used as global logger level. Otherwise, a list of per package levels can be used.
+ E.g.:
+ - `level=info`: the global logger level is set to *info*
+ - `level=warn,tornado=debug`: the global logger level is set to *warn*, the tornado package logger level is set to *debug*
- __stdout-output__: A boolean value that determines whether the Logger should print to standard output.
Valid values are *true* and *false*.
- __file-output-path__: An optional string that defines a file path in the file system. If
