Add goreportcard to README
[![CircleCI](https://circleci.com/gh/prometheus/node_exporter/tree/master.svg?style=shield)][circleci]
[![Docker Repository on Quay](https://quay.io/repository/prometheus/node-exporter/status)][quay]
[![Docker Pulls](https://img.shields.io/docker/pulls/prom/node-exporter.svg?maxAge=604800)][hub]
+[![Go Report Card](https://goreportcard.com/badge/github.com/prometheus/node_exporter)][goreportcard]
Prometheus exporter for hardware and OS metrics exposed by \*NIX kernels, written
in Go with pluggable metric collectors.
@@ -135,3 +136,4 @@ Be aware though that the mountpoint label in various metrics will now have
[hub]: https://hub.docker.com/r/prom/node-exporter/
[circleci]: https://circleci.com/gh/prometheus/node_exporter
[quay]: https://quay.io/repository/prometheus/node-exporter
+[goreportcard]: https://goreportcard.com/report/github.com/prometheus/node_exporter
