README: Add fork note

README.md

@@ -2,6 +2,10 @@
 
 This is a plugin for Trunk Recorder that publishes metrics in Prometheus format.
 
+## Fork note
+
+This uses [a fork of Trunk Recorder](https://github.com/USA-RedDragon/trunk-recorder/tree/main) until [this fix for SoapySDRPlay3](https://github.com/robotastic/trunk-recorder/pull/853) can be merged upstream, so upstream changes may be slightly delayed.
+
 ## Grafana Dashboard
 
 A Grafana dashboard is available [here](https://grafana.com/grafana/dashboards/19959) and also in the `grafana/dashboard.json` file in this repository.