doc: add godoc button

Little detail that makes it easier to find the link to godoc.

Change-Id: Ic3fb467ae23bce3cc2e3801d78221a4187be4462
Reviewed-on: https://go-review.googlesource.com/27915
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/README.md b/README.md
index 0d51417..1643c08 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,7 @@
 # OAuth2 for Go
 
 [![Build Status](https://travis-ci.org/golang/oauth2.svg?branch=master)](https://travis-ci.org/golang/oauth2)
+[![GoDoc](https://godoc.org/golang.org/x/oauth2?status.svg)](https://godoc.org/golang.org/x/oauth2)
 
 oauth2 package contains a client implementation for OAuth 2.0 spec.