README: add goreportcard.com badge to Readme (#16133)

* README: add goreportcard.com badge to Readme

* README: fix double github.com

README.md

@@ -5,6 +5,7 @@ Official golang implementation of the Ethereum protocol.
 [![API Reference](
 https://camo.githubusercontent.com/915b7be44ada53c290eb157634330494ebe3e30a/68747470733a2f2f676f646f632e6f72672f6769746875622e636f6d2f676f6c616e672f6764646f3f7374617475732e737667
 )](https://godoc.org/github.com/ethereum/go-ethereum)
+[![Go Report Card](https://goreportcard.com/badge/github.com/ethereum/go-ethereum)](https://goreportcard.com/report/github.com/ethereum/go-ethereum)
 [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/ethereum/go-ethereum?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
 
 Automated builds are available for stable releases and the unstable master branch.