Document batching behavior. (#295)

README.md

@@ -5,6 +5,10 @@ metrics to Stackdriver. This is based on [this design](docs/design.md).
 
 Custom metrics are a chargeable feature of Stackdriver Monitoring and there could be costs for your custom metrics. For more information on pricing, see [Stackdriver Pricing](https://cloud.google.com/stackdriver/pricing).
 
+The sidecar batches up to
+[200 timeseries per request](https://github.com/Stackdriver/stackdriver-prometheus-sidecar/blob/11096997d11e605190d04fd9f62e323efbfca1c0/stackdriver/client.go#L45)
+(200 is the [limit imposed by the Google Cloud Monitoring API](https://cloud.google.com/monitoring/quotas)).
+
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