README: add Caddy proxy example

[philips]

What's new?

README: It took me a minute to figure out that Pinchflat needed me to "strip_prefix" so maybe this will help others.

What's changed?

Add docs on Caddy configuration

Any other comments?

It took me a minute to figure all of this out so I thought it might help others. No worries if you don't want to merge.

• [x] I am the original author of this code and I am giving it freely to the community and Pinchflat project maintainers

[pyrox0]

I don't need this in my setup. I just have a plain reverse proxy from my caddy machine -> pinchflat, and it just works. This would only be needed if you don't want to run it on a subdomain, but as a subfolder of an existing domain.

[kieraneglin]

After thinking on it, I'm probably going to add the reverse proxy stuff to a dedicated page in the docs rather than the README. I'll leave this PR open as a reference/reminder until I do so!

[philips]

@kieraneglin Yeah, a separate doc makes sense. Thanks

[philips]

@climba03003 thank you for the review. merged your suggestion and rebased.