Add entry to FAQ how to switch javafx cache dirs

[Siedlerchr]

@technobirb you can modify the [JavaOptions] section in the .cfg file. You can also place an additional config file in your system https://bugs.openjdk.org/browse/JDK-8287060

@arnaumas This works also for mac

(make sure to copy JabRef's one with all the options

The other idea is to probably switch the variable for the user directroy:

[JavaOptions]
java-options=-Djavafx.cachedir=/Path/To/Your/direcotry
java-options=-Duser.home=/path/to/my/new/home/dir

Originally posted by @Siedlerchr in #5817