Node mixin: Clarify dashboard dependency on rules (#1475)
Node mixin: Clarify dashboard dependency on rules (#1475)

Following @discordianfish's suggestion
[here](https://github.com/prometheus/node_exporter/issues/1454#issuecomment-524225222).

Signed-off-by: beorn7 <91e32ae1d26bd312b370cf8378a5a83accca90a0@grafana.com>
siavashs/node_exporter,prometheus/node_exporter,siavashs/node_exporter,prometheus/node_exporter,prometheus/node_exporter,siavashs/node_exporter,siavashs/node_exporter