[Feature Request] add README explanation.

Add section in README.md for newcomers like myself about how it "compares"/"complements"/"overlaps" with existing "alternatives".

(.codeeditor, java-lint, ...)

Start with README.md and review how the project should be compared with or distinguished from the alternatives mentioned in the issue: .codeeditor and java-lint. Add a newcomer-oriented section explaining how Prettier Java compares, complements, or overlaps with those alternatives; the README should clearly communicate when each option is relevant.

索引モデルが issue の本文から書いたものです。