chore: update readme for instana resource detector

Update README of Instana Resource Detector to clarify the source of the data.

Part Of #2055

Signed-off-by: maryliag <marylia.gutierrez@grafana.com>

detectors/node/opentelemetry-resource-detector-instana/README.md

@@ -42,6 +42,15 @@ const sdk = new NodeSDK({
 sdk.start()
 ```
 
+## Available detectors
+
+### Instana Agent Detector
+
+| Attribute           | Description                                                              |
+|---------------------|--------------------------------------------------------------------------|
+| process.pid         | Value of `pid` from `/com.instana.plugin.nodejs.discovery` request       |
+| service.instance.id | Value of `agentUuid` from `/com.instana.plugin.nodejs.discovery` request |
+
 ## Useful links
 
 - For more information about Instana Agent, visit: <https://www.ibm.com/docs/en/instana-observability/current?topic=instana-host-agent>