Add suggestion for running with GTK 2 in the Readme

[Thihup]

Hi. Thanks for this application.

I'm using Manjaro Linux and I've hit on some weird edge case: running the application with java -jar JDKMon-linux.jar nothing happens. However, when I run with GTK2, it works just fine: java -Djdk.gtk.version=2 -jar JDKMon-linux.jar

I'd like to suggest adding this property in the ReadMe, in case someone tries to run your application and have the same issue.

[HanSolo]

Hi there and thx for the heads up...will directly add it to the readme... 👍🏻