Added go report card bedge
<img src="https://img.shields.io/badge/golang-%3E%3D%201.9.2-orange.svg"/>
<a href="https://gitter.im/spacemesh-os/Lobby"><img src="https://img.shields.io/badge/gitter-%23spacemesh--os-blue.svg"/></a>
<a href="https://spacemesh.io"><img src="https://img.shields.io/badge/madeby-spacemeshos-blue.svg"/></a>
+[![Go Report Card](https://goreportcard.com/badge/github.com/spacemeshos/go-spacemesh)](https://goreportcard.com/report/github.com/spacemeshos/go-spacemesh)
<a href="https://travis-ci.org/spacemeshos/go-spacemesh"><img src="https://api.travis-ci.org/spacemeshos/go-spacemesh.svg?branch=master"/></a>
</p>
