[receiver/kafkareceiver] Update README (#3665)
[receiver/kafkareceiver] Update README (#3665)

The receiver supports metrics, traces, and logs via OTLP.
open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector