Docs / readme improvement: clarify behavior with reverse proxied backends

[binaryfire]

Hi guys

Would it be possible to add some information to the readme re: how this module deals with reverse proxied content? Specifically:

• If the upstream content is already compressed with Brotli, does this module decompress+recompress the content or does it pass it through unmodified? Hopefully the latter since I'm trying to avoid unnecessary costly compression steps.

• Is there anything we need to configure differently when dealing with proxied backends? There doesn't seem to be a brotli_proxied directive (similar to gzip_proxied).

I've spent quite a while searching for answers to this but there's very little info out there. Could someone help? I think covering it in the readme would help a lot of people.