update book code

eBook/examples/chapter_12/io_interfaces.go

@@ -0,0 +1,18 @@
+// interfaces being used in the GO-package fmt
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+func main() {
+	// unbuffered
+	fmt.Fprintf(os.Stdout, "%s\n", "hello world! - unbuffered")
+	// buffered: os.Stdout implements io.Writer
+	buf := bufio.NewWriter(os.Stdout)
+	// and now so does buf.
+	fmt.Fprintf(buf, "%s\n", "hello world! - buffered")
+	buf.Flush()
+}