Add GoDoc reference badge, fix 404 API link (#26)

* Add GoDoc reference badge, fix 404 API link

* Point to GoDoc via Firebase url

Point to GoDoc via https://godoc.org/firebase.google.com/go

Author: ernsheong

README.md

@@ -1,4 +1,5 @@
 [![Build Status](https://travis-ci.org/firebase/firebase-admin-go.svg?branch=master)](https://travis-ci.org/firebase/firebase-admin-go)
+[![GoDoc](https://godoc.org/firebase.google.com/go?status.svg)](https://godoc.org/firebase.google.com/go)
 
 # Firebase Admin Go SDK
 
@@ -41,7 +42,7 @@ requests, code review feedback, and also pull requests.
 ## Documentation
 
 * [Setup Guide](https://firebase.google.com/docs/admin/setup/)
-* [API Reference](https://firebase.google.com/docs/reference/admin/go/)
+* [API Reference](https://godoc.org/firebase.google.com/go)
 
 
 ## License and Terms
@@ -50,4 +51,4 @@ Firebase Admin Go SDK is licensed under the
 [Apache License, version 2.0](http://www.apache.org/licenses/LICENSE-2.0).
 
 Your use of Firebase is governed by the
-[Terms of Service for Firebase Services](https://firebase.google.com/terms/).
+[Terms of Service for Firebase Services](https://firebase.google.com/terms/).