io/ioutil: add example for ReadFile

Change-Id: I062ec0606a741c8aa1b74f3f4b4e4fd47f76fed5
Reviewed-on: https://go-review.googlesource.com/48775Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/io/ioutil/example_test.go

@@ -69,3 +69,15 @@ func ExampleTempFile() {
 		log.Fatal(err)
 	}
 }
+
+func ExampleReadFile() {
+	content, err := ioutil.ReadFile("testdata/hello")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("File contents: %s", content)
+
+	// Output:
+	// File contents: Hello, Gophers!
+}

src/io/ioutil/testdata/hello

+Hello, Gophers!