Mention the default listening port in the README.
Mention the default listening port in the README.

Related to https://github.com/prometheus/node_exporter/pull/1948

Signed-off-by: Ben Kochie <9bbf917dc63fbaa40c67921332d26cf16a50db75@gmail.com>

siavashs/node_exporter,prometheus/node_exporter,siavashs/node_exporter,siavashs/node_exporter,prometheus/node_exporter,prometheus/node_exporter,siavashs/node_exporter