Documentation for PROXY protocol is missing

While searching for a way to forward the client IP I found #22 (issue: #13) which has already been released, but the README doesn't contain any info about it.
First of all, the usage code block would need something like this:
```
-use-proxy-protocol
Send PROXY protocol to backend servers (env USE_PROXY_PROTOCOL)
```
And maybe some additional info on configuring the actual minecraft servers to accept the proxy protocol.
(I realize that this might be vastly different between server "flavors", but at least a general hint in the right direction would be helpful anyway)
The way I have it now, I can't connect to a fabric 1.18.1 server behind mc-router when I enable the proxy protocol, but this might be a config error.

Start with the README usage code block and the released PROXY protocol work referenced by issues #22 and #13. Document the USE_PROXY_PROTOCOL option and add a general hint for configuring Minecraft servers, including the reported Fabric 1.18.1 case; done means readers can find the option and understand the server-side setup direction.

Written by the indexing model from the issue text.