nacl/secretbox: add Seal, Open example

I read the docs and wasn't sure how to use the package - what is
the best way to generate a nonce? Do I need to use the same nonce
for encrypting and decrypting the same message? I read through some
code that used this package, and the tests, found the answers I was
looking for, and used them to generate an example, which hopefully
will be helpful to other users of this package.

Change-Id: I24f9682fecb6632f34b4d8d13fed6eb002993677
Reviewed-on: https://go-review.googlesource.com/26810
Reviewed-by: Adam Langley <agl@golang.org>

nacl/secretbox/example_test.go

@@ -0,0 +1,47 @@
+package secretbox
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"fmt"
+	"io"
+)
+
+func Example() {
+	// Load your secret key from a safe place and reuse it across multiple
+	// Seal calls. (Obviously don't use this example key for anything
+	// real.) If you want to convert a passphrase to a key, use a suitable
+	// package like bcrypt or scrypt.
+	secretKeyBytes, err := hex.DecodeString("6368616e676520746869732070617373776f726420746f206120736563726574")
+	if err != nil {
+		panic(err)
+	}
+
+	var secretKey [32]byte
+	copy(secretKey[:], secretKeyBytes)
+
+	// You must use a different nonce for each message you encrypt with the
+	// same key. Since the nonce here is 192 bits long, a random value
+	// provides a sufficiently small probability of repeats.
+	var nonce [24]byte
+	if _, err := io.ReadFull(rand.Reader, nonce[:]); err != nil {
+		panic(err)
+	}
+
+	// This encrypts "hello world" and appends the result to the nonce.
+	encrypted := Seal(nonce[:], []byte("hello world"), &nonce, &secretKey)
+
+	// When you decrypt, you must use the same nonce and key you used to
+	// encrypt the message. One way to achieve this is to store the nonce
+	// alongside the encrypted message. Above, we stored the nonce in the first
+	// 24 bytes of the encrypted text.
+	var decryptNonce [24]byte
+	copy(decryptNonce[:], encrypted[:24])
+	decrypted, ok := Open([]byte{}, encrypted[24:], &decryptNonce, &secretKey)
+	if !ok {
+		panic("decryption error")
+	}
+
+	fmt.Println(string(decrypted))
+	// Output: hello world
+}