os: add OpenFile example for appending data

Fixes #19329.

Change-Id: I6d8bb112a56d751a6d3ea9bd6021803cb9f59234
Reviewed-on: https://go-review.googlesource.com/37619Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/example_test.go

@@ -21,6 +21,20 @@ func ExampleOpenFile() {
 	}
 }
 
+func ExampleOpenFile_append() {
+	// If the file doesn't exist, create it, or append to the file
+	f, err := os.OpenFile("access.log", os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if _, err := f.Write([]byte("appended some data\n")); err != nil {
+		log.Fatal(err)
+	}
+	if err := f.Close(); err != nil {
+		log.Fatal(err)
+	}
+}
+
 func ExampleChmod() {
 	if err := os.Chmod("some-filename", 0644); err != nil {
 		log.Fatal(err)