Update README.md

README.md

@@ -44,6 +44,8 @@ You can "copy as cURL" in your browser and then prepend that with `plt` to throw
 For even better performance you can use `plt curl --fast` that will employ awesome [fasthttp](https://github.com/valyala/fasthttp)
 as transport. This mode lacks detailed breakdown of latencies, but can push request rate to the limit.
 
+If the server is wrapped with Envoy proxy, upstream latency distribution will be collected from the values of [`X-Envoy-Upstream-Service-Time`](https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_filters/router_filter#x-envoy-upstream-service-time) response header.
+
 ## Example
 
 ```bash
@@ -150,4 +152,4 @@ Bytes read 83600000
 Bytes written 18600000
 [200]
 {"id":1,"goal":"enjoy!","deadline":"2020-05-24T21:00:54.998Z","createdAt":"2020-05-24T23:00:56.656017059+02:00"}
-```
+```