log: add example

Fixes #6028.

R=golang-dev, bradfitz, adg, r
CC=golang-dev
https://golang.org/cl/43320043

src/pkg/log/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 log_test
+
+import (
+	"bytes"
+	"fmt"
+	"log"
+)
+
+func ExampleLogger() {
+	var buf bytes.Buffer
+	logger := log.New(&buf, "logger: ", log.Lshortfile)
+	logger.Print("Hello, log file!")
+
+	fmt.Print(&buf)
+	// Output:
+	// logger: example_test.go:15: Hello, log file!
+}