container/list: add package example

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/7306078

src/pkg/container/list/example_test.go

+// Copyright 2013 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 list_test
+
+import (
+	"container/list"
+	"fmt"
+)
+
+func Example() {
+	// Create a new list and put some numbers in it.
+	l := list.New()
+	e4 := l.PushBack(4)
+	e1 := l.PushFront(1)
+	l.InsertBefore(3, e4)
+	l.InsertAfter(2, e1)
+
+	// Iterate through list and and print its contents.
+	for e := l.Front(); e != nil; e = e.Next() {
+		fmt.Println(e.Value)
+	}
+
+	// Output:
+	// 1
+	// 2
+	// 3
+	// 4
+}