net/smtp: add examples

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/8274046

src/pkg/net/smtp/example_test.go

+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package smtp_test
+
+import (
+	"fmt"
+	"log"
+	"net/smtp"
+)
+
+func Example() {
+	// Connect to the remote SMTP server.
+	c, err := smtp.Dial("mail.example.com:25")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Set the sender and recipient first
+	if err := c.Mail("sender@example.org"); err != nil {
+		log.Fatal(err)
+	}
+	if err := c.Rcpt("recipient@example.net"); err != nil {
+		log.Fatal(err)
+	}
+
+	// Send the email body.
+	wc, err := c.Data()
+	if err != nil {
+		log.Fatal(err)
+	}
+	_, err = fmt.Fprintf(wc, "This is the email body")
+	if err != nil {
+		log.Fatal(err)
+	}
+	err = wc.Close()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Send the QUIT command and close the connection.
+	err = c.Quit()
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+func ExamplePlainAuth() {
+	// Set up authentication information.
+	auth := smtp.PlainAuth("", "user@example.com", "password", "mail.example.com")
+
+	// Connect to the server, authenticate, set the sender and recipient,
+	// and send the email all in one step.
+	to := []string{"recipient@example.net"}
+	msg := []byte("This is the email body.")
+	err := smtp.SendMail("mail.example.com:25", auth, "sender@example.org", to, msg)
+	if err != nil {
+		log.Fatal(err)
+	}
+}