Node mixin: Clarify dashboard dependency on rules
Following suggestion
[here](https://github.com/prometheus/node_exporter/issues/1454#issuecomment-524225222).
@@ -39,6 +39,9 @@ for Grafana:
$ make dashboards_out
```
+Note that some of the generated dashboards require recording rules specified in
+the previously generated `node_rules.yaml`.
+
For more advanced uses of mixins, see
https://github.com/monitoring-mixins/docs.
