crypto/sha256: add examples for New, Sum256

The goal for these examples is to show how to mirror the
functionality of the sha256sum Unix utility, a common checksumming
tool, using the Go standard library.

Add a newline at the end of the input, so users will get the same
output if they type `echo 'hello world' | sha256sum`, since the
builtin shell echo appends a newline by default. Also use hex output
(instead of the shorter base64) since this is the default output
encoding for shasum/sha256sum.

Change-Id: I0036874b3cc5ba85432bfcb86f81b51c4e0238fd
Reviewed-on: https://go-review.googlesource.com/24868Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/crypto/sha256/example_test.go

+// Copyright 2016 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 sha256_test
+
+import (
+	"crypto/sha256"
+	"fmt"
+)
+
+func ExampleSum256() {
+	sum := sha256.Sum256([]byte("hello world\n"))
+	fmt.Printf("%x", sum)
+	// Output: a948904f2f0f479b8f8197694b30184b0d2ed1c1cd2a1ec0fb85d299a192a447
+}
+
+func ExampleNew() {
+	h := sha256.New()
+	h.Write([]byte("hello world\n"))
+	fmt.Printf("%x", h.Sum(nil))
+	// Output: a948904f2f0f479b8f8197694b30184b0d2ed1c1cd2a1ec0fb85d299a192a447
+}