The topic says that this library is Reflection-based...
Which means this library uses Reflections (the library).
But its bundled onto 1 jar, so you have the reflections library if you load this lib in.
What I am missing is some hint how to use your library to make your game moddable. It looks like it's just a wrapper for getting annotated classes. What does it provide on top of Reflections?
I would at least expect an api to manage mods inside a folder and to load, enable or disable them dynamically. Maybe also some Resource abstraction to help exchanging textures and sounds in a running game. Sources, javadocs and examples would be nice, too.
You did a fat jar with all dependencies. This might introduce conflicts with other versions of the included dependencies used by a game. Also I don't know, if the license of Refllections (or some of the dependencies) allows rebundling like this...
All in all a good idea, but not ready for prime time. Keep it up nonetheless!