note that Gerrit is used for reviews on README

Fixes golang/oauth2#217.

Change-Id: I251a74b9b26f6b911333bc5d31d0a96a5734ea60
Reviewed-on: https://go-review.googlesource.com/36914
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/README.md b/README.md
index 1643c08..b0ddf3c 100644
--- a/README.md
+++ b/README.md
@@ -63,3 +63,12 @@
 		client.Get("...")
 	}
 
+## Contributing
+
+We appreciate your help!
+
+To contribute, please read the contribution guidelines:
+	https://golang.org/doc/contribute.html
+
+Note that the Go project does not use GitHub pull requests but
+uses Gerrit for code reviews. See the contribution guide for details.