fix(readme): document QUIC and HTTP/3 compatibility caveats

readme.md

@@ -1111,6 +1111,10 @@ SmartProxy searches for the Rust binary in this order:
 5. Local dev build (`./rust/target/release/rustproxy`)
 6. System PATH (`rustproxy`)
 
+### QUIC / HTTP3 Caveats
+- **GREASE frames are disabled.** The underlying h3 crate sends [GREASE frames](https://www.rfc-editor.org/rfc/rfc9114.html#frame-reserved) by default to test protocol extensibility. However, some HTTP/3 clients and servers don't properly ignore unknown frame types, causing 400/500 errors or stream hangs ([h3#206](https://github.com/hyperium/h3/issues/206)). SmartProxy disables GREASE on both the server side (for incoming H3 requests) and the client side (for H3 backend connections) to maximize compatibility.
+- **HTTP/3 is pre-release.** The h3 ecosystem (h3 0.0.8, h3-quinn 0.0.10, quinn 0.11) is still pre-1.0. Expect rough edges.
+
 ### Performance Tuning
 - ✅ Use NFTables forwarding for high-traffic routes (Linux only)
 - ✅ Enable connection keep-alive where appropriate