strings: add a example for Compare func

Add a example for string.Compare that return the three possible results.

Change-Id: I103cf39327c1868fb249538d9e22b11865ba4b70
Reviewed-on: https://go-review.googlesource.com/49011
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/strings/example_test.go

@@ -23,6 +23,16 @@ func ExampleFieldsFunc() {
 	// Output: Fields are: ["foo1" "bar2" "baz3"]
 }
 
+func ExampleCompare() {
+	fmt.Println(strings.Compare("a", "b"))
+	fmt.Println(strings.Compare("a", "a"))
+	fmt.Println(strings.Compare("b", "a"))
+	// Output:
+	// -1
+	// 0
+	// 1
+}
+
 func ExampleContains() {
 	fmt.Println(strings.Contains("seafood", "foo"))
 	fmt.Println(strings.Contains("seafood", "bar"))