add godoc

Author: c-wind

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://travis-ci.org/dearcode/candy.svg?branch=master)](https://travis-ci.org/dearcode/candy)
 [![Go Report Card](https://goreportcard.com/badge/github.com/dearcode/candy)](https://goreportcard.com/report/github.com/dearcode/candy)
+[![GoDoc](https://godoc.org/github.com/dearcode/candy?status.svg)](https://godoc.org/github.com/dearcode/candy)
 
 
 ## 项目背景