json: Add package comment

R=golang-dev, r2, r
CC=golang-dev
https://golang.org/cl/2149047

src/pkg/json/encode.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// The json package implements encoding and decoding of JSON objects as
+// defined in RFC 4627.
 package json
 
 import (