[chore][processor/servicegraph] add deprecation note to README

processor/servicegraphprocessor/README.md

@@ -13,6 +13,8 @@
 [sumo]: https://github.com/SumoLogic/sumologic-otel-collector
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+The service graph processor is deprecated and will be removed in future versions. Use the [service graph connector](https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/connector/servicegraphconnector/README.md) instead.
+
 The service graphs processor is a traces processor that builds a map representing the interrelationships between various services in a system.
 The processor will analyse trace data and generate metrics describing the relationship between the services.
 These metrics can be used by data visualization apps (e.g. Grafana) to draw a service graph.