Create jitpack.yml

[threethan]

This is a very small change which allows fxthemes to be used through jitpack. I know that it's available through maven central, but that version is out of date, and as far as I'm aware there's no downside to supporting both repositories.

[koppor]

@dukke I hae experiences in jitpack - and I would really like to see this - this would allow for using the latest commits without any publishing at maven central.