diff --git a/README.md b/README.md
index 554d2c6..bd9d38a 100644
--- a/README.md
+++ b/README.md
@@ -123,6 +123,8 @@ When your application cannot support Lobbyd's API it can be configured via callb
 
 The script runs under the same user as lobbyd. When lobbyd starts first thirty seconds (CALLBACK_FIRST_RUN_DELAY) is ignored and then the script is run for first time. After these thirty seconds everything runs in loop based on the changes in the network.
 
+All current discovery packets are passed to the callback script via standard input. It's basically the same input you get if you run `lobbyctl discoveries`.
+
 ### Service discovery for Prometheus
 
 Lobbyd has an API endpoint that returns list of targets for [Prometheus's HTTP SD config](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#http_sd_config). That