docs(helm): restore LMCache dashboard explanation in Observability section

The LMCache dashboard's six fields were documented in the old
observability/README.md, which is now deprecated and points readers to
helm/README.md#observability for this content. That content never
made it over during the helm chart monitoring refactor (#860), even
though the dashboard itself is still shipped and auto-provisioned via
helm/templates/dashboards.yaml.

Signed-off-by: Sam <samhoss93@gmail.com>

Author: samhoss93

helm/README.md

@@ -459,6 +459,10 @@ kubectl port-forward svc/<release-name>-grafana 8080:80
 
 Open the webpage at `http://<IP of your node>:8080` to access the Grafana web page. The default user name is `admin` and the password can be configured in the values (default is generated by helm and stored in a secret `<release-name>-grafana`).
 
+### LMCache Dashboard
+
+If you use the LMCache image in the production stack, the chart includes a dedicated LMCache dashboard (provisioned automatically alongside the main vLLM dashboard when `grafanaDashboards.enabled` is set to `true`). It contains six fields showing the benefits of CPU offloading: Average time to first token (sec), Cache hit rate (%) in last 1 minute, LMCache retrieve speed (K Tokens / sec), Local CPU cache usage (GB), Number of requested tokens in total, and Number of hit tokens in total.
+
 ### Use Prometheus Adapter to export vLLM metrics
 
 The vLLM router can export metrics to Prometheus using the [Prometheus Adapter](https://github.com/prometheus-community/helm-charts/tree/main/charts/prometheus-adapter).