docs: Note how to add a content coding

Records the extension points for anyone who wants zstd or brotli, and
why deflate is not worth adding.

Author: thced

README.md

@@ -523,6 +523,16 @@ advertising the uncoded length.
   renderer and are well under any sensible threshold
 - per-route or per-operation opt-out
 
+**Adding a coding (e.g. zstd).** Neither brotli nor zstd is in the JDK, so either needs a
+dependency — declare it `optional` so consumers inherit nothing, and detect it with a
+`Class.forName` probe the way Gson is detected. `deflate` needs no dependency but is only ~12 bytes
+better than gzip on any payload, which is why it is not here. The four extension points are
+`ContentEncodingHeader.Coding` (request codings), `RequestBodyReader.inflate` (decode),
+`ResponseCompression` (encode) and the `Content-Encoding` the renderer sets. The one real change is
+`AcceptEncodingHeader.acceptsGzip`, which answers a yes/no question and would have to return the
+client's highest-weighted supported coding instead — the q-value parsing it already does is what
+that ordering needs.
+
 ### Graceful shutdown
 
 `OpenApiServer` exposes `stop(int delaySeconds)` for explicit shutdown that waits up to the given