(docs) Update README with cleaned junk categories

[avatGit]

What does this PR do?

This PR addresses the issue requesting a documented list of the junk categories covered by sifty junk scan inside the README. It adds a clear bulleted list explaining each of the 11 core categories and highlights which one is disabled by default.
Closes #21

Notes for the reviewer

• 11 Categories: For now, this list strictly documents the current 11 baseline categories from main.
• Discord Cache Note: I am intentionally leaving out the discord-cache category from this list for the moment to avoid dependency conflicts with my other pending PR((feat) add Discord cache category to junk cleaner #24 ). Once the Discord cache PR is merged into main, I will gladly open a quick follow-up commit/PR to bump the count to 12 and add it to this list.

[github-actions]

Thanks for your first pull request to Sifty! A couple of things that help it land quickly: Sifty deletes files, so all deletion must go through safety.trash() and destructive paths default to dry-run. Please make sure pytest is green (especially tests/test_safety.py) and that the PR title starts with a type tag like (feat) or (fix). See CONTRIBUTING.md for the full checklist. A maintainer will review shortly.