oauth2: add reference to clientcredentials package

This adds a reference to the golang.org/x/oauth2/clientcredentials
package in the docstring for oauth2.Config.

Change-Id: Iee0f36e07fda76c25b9f79e0db3807867a39fab2
Reviewed-on: https://go-review.googlesource.com/25387
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>
diff --git a/oauth2.go b/oauth2.go
index d54ee26..0505263 100644
--- a/oauth2.go
+++ b/oauth2.go
@@ -37,6 +37,8 @@
 
 // Config describes a typical 3-legged OAuth2 flow, with both the
 // client application information and the server's endpoint URLs.
+// For the client credentials 2-legged OAuth2 flow, see the clientcredentials
+// package (https://golang.org/x/oauth2/clientcredentials).
 type Config struct {
 	// ClientID is the application's ID.
 	ClientID string