Document common "I see no data" question

This comes up frequently, because it is confusing unless you understand the
intricacies of the Prometheus data and query model, CloudWatch metrics
convergence, and the relevant settings.

Fixes #580.
Explanation adapted from #483 (comment).

Signed-off-by: Matthias Rampke <[email protected]>

README.md

@@ -145,6 +145,19 @@ For certain metrics which update relatively rarely, such as from S3,
 a timestamp. This is as the true timestamp from CloudWatch could be so old that
 Prometheus would reject the sample.
 
+#### FAQ: I can see the metrics in `/metrics` but not in the Prometheus web console
+
+The metrics will be visible in Prometheus if you look more than `delay_seconds` in the past.
+Try the graph view.
+
+This is an unfortunate result of a fundamental mismatch between CloudWatch and Prometheus.
+CloudWatch metrics converge over time, that is, the value at time `T` can change up to some later time `T+dT`.
+Meanwhile, Prometheus assumes that once it has scraped a sample, that is the truth, and the past does not change.
+
+To compensate for this, by default the exporter [delays fetching metrics](https://github.com/prometheus/cloudwatch_exporter/blob/master/README.md#timestamps), that is, it only asks for data 10 minutes later, when _almost_ all AWS services have converged.
+It also reports to Prometheus that this sample is from the past.
+Because Prometheus, for an instant request, only looks back 5 minutes, it never sees any data "now".
+
 ### Special handling for certain DynamoDB metrics
 
 The DynamoDB metrics listed below break the usual CloudWatch data model.