time: example in doc for time.AppendFormat func

The simple example would contribute to better understanding
what function does.

Change-Id: I36a2952df8b0e1762ec0cd908a867c457f39366e
Reviewed-on: https://go-review.googlesource.com/75970Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/time/example_test.go

@@ -569,3 +569,14 @@ func ExampleTime_Sub() {
 	// Output:
 	// difference = 12h0m0s
 }
+
+func ExampleTime_AppendFormat() {
+	t := time.Date(2017, time.November, 4, 11, 0, 0, 0, time.UTC)
+	text := []byte("Time: ")
+
+	text = t.AppendFormat(text, time.Kitchen)
+	fmt.Println(string(text))
+
+	// Output:
+	// Time: 11:00AM
+}