strings: add Fields example

R=golang-dev, rsc, r
CC=golang-dev
https://golang.org/cl/5629043

src/pkg/strings/example_test.go

+// Copyright 2012 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 strings_test
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Fields are: ["foo" "bar" "baz"]
+func ExampleFields() {
+	fmt.Printf("Fields are: %q", strings.Fields("  foo bar  baz   "))
+}