doc: Add --use-nginx-conf-files usage example to README for better visibility

[ingen2]

Hi 👋

Thanks for the great work on this project!
I’ve noticed that the --use-nginx-conf-files flag for fresh-start.sh is very useful, especially when customizing Nginx settings like max upload size. However, it's not mentioned in the README, and I often find myself searching through issues or code to remember the correct usage.

It would be very helpful to add a short example to the README near this section:

cd proxy/bin && ./fresh-start.sh --yes --skip-docker-image-check -e your_email@domain

Maybe something like:

If you want to use custom nginx config files (e.g. for upload size)

cd proxy/bin && ./fresh-start.sh --yes --skip-docker-image-check -e your_email@domain --use-nginx-conf-files

This would make the feature more visible and accessible for others who need to tweak Nginx settings.
Thanks again!