bytes: add example for Len function of Reader

Change-Id: If7ecdc57f190f647bfc673bde8e66b4ef12aa906
Reviewed-on: https://go-review.googlesource.com/64190Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/bytes/example_test.go

@@ -412,3 +412,13 @@ func ExampleToLower() {
 	fmt.Printf("%s", bytes.ToLower([]byte("Gopher")))
 	// Output: gopher
 }
+
+func ExampleReader_Len() {
+	fmt.Println(bytes.NewReader([]byte("")).Len())
+	fmt.Println(bytes.NewReader([]byte("Hi!")).Len())
+	fmt.Println(bytes.NewReader([]byte("Hello Gopher!")).Len())
+	// Output:
+	// 0
+	// 3
+	// 13
+}