Update README.md
Add go report card.
-Longhorn Manager [![Build Status](https://drone8.rancher.io/api/badges/rancher/longhorn-manager/status.svg)](https://drone8.rancher.io/rancher/longhorn-manager)
+Longhorn Manager
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+[![Build Status](https://drone8.rancher.io/api/badges/rancher/longhorn-manager/status.svg)](https://drone8.rancher.io/rancher/longhorn-manager) [![Go Report Card](https://goreportcard.com/badge/github.com/rancher/longhorn-manager)](https://goreportcard.com/report/github.com/rancher/longhorn-manager)
Manager for Longhorn.
