text: Title: Document where `log-<index>.js` files created by `--splitlog` are saved
  Body: Currently, when using `--splitlog` command, it will generate additional JS files. When the output.xml is large, a bunch of *.js files are generated, which makes the current directory flooded.
  
  **Needs:**
  Ability to group them into a sub-folder to tidy up the directory.
  
  E.g
  `rebot --splitlog ./resource ./output.xml`