godoc: add dummy playground.js to silence godoc warning at start-up

Fixes #2173.

R=r, rsc
CC=golang-dev
https://golang.org/cl/4941044

doc/play/playground.js

+// Copyright 2011 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.
+
+// A dummy playground.js file to silence godoc errors
+// when accessing doc/root.html.