jws: add notice that the package might be removed

This package is not a general-use JWS implementation and should live
under internal. For now, just add a warning that no new users should
depend on it.

Updates #196.

Change-Id: I0eef273c8327a5ad26eb33a4425afcadca23494b
Reviewed-on: https://go-review.googlesource.com/27692
Reviewed-by: Brad Fitzpatrick <[email protected]>

jws/jws.go

@@ -2,8 +2,16 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package jws provides encoding and decoding utilities for
-// signed JWS messages.
+// Package jws provides a partial implementation
+// of JSON Web Signature encoding and decoding.
+// It exists to support the golang.org/x/oauth2 package.
+//
+// See RFC 7515.
+//
+// Deprecated: this package is not intended for public use and might be
+// removed in the future. It exists for internal use only.
+// Please switch to another JWS package or copy this package into your own
+// source tree.
 package jws // import "golang.org/x/oauth2/jws"
 
 import (