encoding/base64: add package-level example

Fixes #13011.

Change-Id: Ia4c67880fca83f4298ff6bb1b217ec26c8c83427
Reviewed-on: https://go-review.googlesource.com/17231Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/base64/example_test.go

@@ -12,6 +12,21 @@ import (
 	"os"
 )
 
+func Example() {
+	msg := "Hello, 世界"
+	encoded := base64.StdEncoding.EncodeToString([]byte(msg))
+	fmt.Println(encoded)
+	decoded, err := base64.StdEncoding.DecodeString(encoded)
+	if err != nil {
+		fmt.Println("decode error:", err)
+		return
+	}
+	fmt.Println(string(decoded))
+	// Output:
+	// SGVsbG8sIOS4lueVjA==
+	// Hello, 世界
+}
+
 func ExampleEncoding_EncodeToString() {
 	data := []byte("any + old & data")
 	str := base64.StdEncoding.EncodeToString(data)