Your message is exactly how you should get started :-)
As mentioned in the blog post, even just the purpose of the library is beyond beginner Java level. Are you already familiar with the technique of using immutable objects? E.g. what it's good for, what the trade-offs are, etc? It will help when contributing if you understand the concepts, and see how Mutability Detector 'fits in'. If you don't know that's fine, we can discuss here, and you can feel free to ask questions. Also, how familiar are you with test driven development (TDD) and continuous integration (CI)? That will also be helpful to understand.
So, first step is to give an understanding of why immutability is a useful technique. Do you know why it's useful?
Best regards,
Graham A
(P.S. very pleased you're interested!)