add doc for --log-file

docs/usage.md

@@ -176,6 +176,14 @@ To split the data CSV into training, validation, and testing CSVs, the `gandlf s
   -o $output_dir # the output directory to save the split data
 ```
 
+### Using the `--log-file` parameter
+By default, only the `info` and `error` logs will be **displayed** in the console and
+the log file will be **saved** in `$(home)/.gandlf/<timestamp>.log`.
+
+Also, you can use the `--log-file` and provide the file that you want to save the logs
+```bash
+(venv_gandlf) $> gandlf <command> --log-file <log_file_path>
+```
 
 ## Customize the Training