Add --report-file option to readme

Author: per1234

README.md

@@ -41,6 +41,7 @@ paths    Paths containing Arduino projects to check. If a path is a folder, it w
     --log-level string            Messages with this level and above will be logged. Valid levels are:
                                   trace, debug, info, warn, error, fatal, panic
     --format string               The output format, can be {text|json}. (default "text")
+    --report-file string          Save a report on the checks to this file.
 -v, --verbose                     Print the logs on the standard output.
 ```