logging: add 'By Config File' section for log path configuration

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

deployment/logging.md

@@ -148,12 +148,28 @@ end
 
 ## Output to Log File
 
+### By Command Line Option
+
 By default, Fluentd outputs to the standard output. Use `-o` command line option to specify the file instead:
 
 ```text
 $ fluentd -o /path/to/log_file
 ```
 
+### By Config File
+
+You can also configure the log file path using the `<log>` directive under `<system>`:
+
+```text
+<system>
+  <log>
+    path /path/to/log_file
+  </log>
+</system>
+
+$ fluentd -o /path/to/log_file
+```
+
 ### Log Rotation Setting
 
 By default, Fluentd does not rotate log files. You can configure this behavior via system-config after v1.13.0.