add badges

README.md

@@ -1,4 +1,9 @@
+[![Sourcegraph](https://sourcegraph.com/github.com/json-iterator/go/-/badge.svg)](https://sourcegraph.com/github.com/json-iterator/go?badge)
+[![GoDoc](http://img.shields.io/badge/go-documentation-blue.svg?style=flat-square)](http://godoc.org/github.com/json-iterator/go)
+[![Build Status](https://travis-ci.org/json-iterator/go.svg?branch=master)](https://travis-ci.org/json-iterator/go)
+[![codecov](https://codecov.io/gh/json-iterator/go/branch/master/graph/badge.svg)](https://codecov.io/gh/json-iterator/go)
 [![rcard](https://goreportcard.com/badge/github.com/json-iterator/go)](https://goreportcard.com/report/github.com/json-iterator/go)
+[![License](http://img.shields.io/badge/license-mit-blue.svg?style=flat-square)](https://raw.githubusercontent.com/json-iterator/go/master/LICENSE)
 
 A high-performance 100% compatible drop-in replacement of "encoding/json"