Document how to check out asf-site on case-insensitive file systems

I lost some time trying to figure out why git kept showing files in the asf-site branch as modified immediately after checkout, until I finally realized that some paths exist both as lowercase and mixed-case. I think a few additions to the readme can help avoid that kind of friction for others.

Tasks:
- [ ] for macOS, briefly describe how to create a sparse image to clone into, ideally as a terminal command
- [ ] give an example for a shallow clone of the asf-site branch to avoid pulling the entire history