docs: add Maven Central badge to increase libtiled-java visibility

[exoego]

So libtiled-java users can find latest version (and past versions) easily.

[dogboydog]

Just adds a badge, I think it looks good. @bjorn ?

[bjorn]

While it's a nice idea in principle, I'm hesitant about this because libtiled-java has not been properly maintained in a very long time, with the last real updates (apart from some optimizations and dependency version updates) being almost 6 years ago. So I'd prefer to add this badge only to util/java/libtiled-java/README.md for now.

[dogboydog]

Moved to the requested location , looks good

[bjorn]

Thanks @exoego! I see you've also started to modernize libtiled-java in #4325. That's great!

I do think that we should probably move this library into its own repository, like all other such libraries really. But if we do this we'll make sure to do it in a way that keeps its development history.