Add information on using pre-built / contributed dashboards.

Updated (typos and copy clarity etc.) following PR comments.

content/docs/visualization/grafana.md

@@ -61,3 +61,8 @@ legend format string `{{method}} - {{status}}`.
 
 The following shows an example Prometheus graph configuration:
 [![Prometheus graph creation](/assets/grafana_qps_graph.png)](/assets/grafana_qps_graph.png)
+
+### Importing pre-built dashboards from Grafana.net
+Grafana.net maintains [a collection of shared dashboards](https://grafana.net/dashboards) which can be downloaded and used with standalone instances of Grafana.  Use the Grafana.net "Filter" option to browse dashboards for the "Prometheus" datasource only.
+
+You must currently manually edit the downloaded JSON files and correct the "datasource:" entries to reflect the Grafana data source name which you chose for your Prometheus server.  Use the "Dashboards" → "Home" → "Import" option to import the edited dashboard file into your Grafana install.