State that MacPacker implements a Quick Look extension in README

[davidyuk]

In https://github.com/sarensw/MacPacker/blob/main/README.md and https://macpacker.app.

I was looking exactly for a Quick Look extension that supports zip previews. I was surprised to find one after I installed MacPacker. It would be easier to decide if I knew it in advance 🙃 Thanks for the app!

[sarensw]

@davidyuk agreed. Docs are a big topic that I want to extend soon. Thanks for raising the issue.