crypto/rand: Example for Read.

R=adg, remyoudompheng, rsc, r
CC=golang-dev
https://golang.org/cl/6457085

src/pkg/crypto/rand/example_test.go

+// Copyright 2011 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 rand_test
+
+import (
+	"bytes"
+	"crypto/rand"
+	"fmt"
+	"io"
+)
+
+// This example reads 10 cryptographically secure pseudorandom numbers from
+// rand.Reader and writes them to a byte slice.
+func ExampleRead() {
+	c := 10
+	b := make([]byte, c)
+	n, err := io.ReadFull(rand.Reader, b)
+	if n != len(b) || err != nil {
+		fmt.Println("error:", err)
+		return
+	}
+	// The slice should now contain random bytes instead of only zeroes.
+	fmt.Println(bytes.Equal(b, make([]byte, c)))
+
+	// Output:
+	// false
+}