Umineko: No EnableSteamSync.bat equivalent on linux/mac

On the Steam forums, a Mac user asked how to run the `EnableSteamSync.bat` script.

Currently, the only way to enable steam sync on mac or linux is to manually create a linked folder (make a link with `ln` which points `saves` to the `mysav` folder).

I think making a shell script which calls `ln -s mysav saves` (with added checks if the 'saves' folder already exists, as in the current EnableSteamSync batch file) would be enough on linux/mac to get this to work.

The documentation should also be updated to state the OS specific differences. Currently it doesn't say that the `EnableSteamSync.bat` is windows specific.

Look at the existing EnableSteamSync.bat file to understand its logic for checking if the 'saves' folder exists. Create a shell script (likely named EnableSteamSync.sh) that uses ln -s mysav saves with similar checks. Update the documentation to mention OS-specific differences, noting that the .bat file is Windows-only and the .sh script is for Linux/Mac. Test by running the script in a Linux/Mac environment to ensure it creates the symlink correctly.

Written by the indexing model from the issue text.