fmt: add Stringer example

Change-Id: I901f995f8aedee47c48252745816e53192d4b7e4
Reviewed-on: https://go-review.googlesource.com/49090Reviewed-by: Sam Whited <sam@samwhited.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Sam Whited <sam@samwhited.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/fmt/example_test.go

+// Copyright 2017 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 fmt_test
+
+import (
+	"fmt"
+)
+
+// Animal has a Name and an Age to represent an animal.
+type Animal struct {
+	Name string
+	Age  uint
+}
+
+// String makes Animal satisfy the Stringer interface.
+func (a Animal) String() string {
+	return fmt.Sprintf("%v (%d)", a.Name, a.Age)
+}
+
+func ExampleStringer() {
+	a := Animal{
+		Name: "Gopher",
+		Age:  2,
+	}
+	fmt.Println(a)
+	// Output: Gopher (2)
+}