box: add box.com OAuth endpoint

More info [here][1].

[1]: https://box-content.readme.io/docs/oauth-20

box/box.go

@@ -0,0 +1,16 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package box provides constants for using OAuth2 to access box.com.
+package box // import "golang.org/x/oauth2/box"
+
+import (
+	"golang.org/x/oauth2"
+)
+
+// Endpoint is box.com's OAuth 2.0 endpoint.
+var Endpoint = oauth2.Endpoint{
+	AuthURL:  "https://account.box.com/api/oauth2/authorize",
+	TokenURL: "https://api.box.com/oauth2/token",
+}