Metrics example (open-telemetry#266)

Author: HudsonHumphries

examples/metrics_simple/README.md

@@ -65,3 +65,5 @@ See [CONTRIBUTING.md](../../CONTRIBUTING.md) for instructions on building and ru
 [SDK Design](https://github.com/open-o11y/otel-docs/blob/master/cpp-metrics/sdk-design.md)
 
 [OStreamExporters Design](https://github.com/open-o11y/otel-docs/blob/master/exporter/ostream/ostream-exporter-design.md)
+
+[OpenTelemetry C++ Metrics Overview](https://github.com/open-o11y/docs/blob/master/cpp-metrics/README.md)