doc: added The Go image package article

Orignally published on The Go Programming Language, September 21, 2011.

http://blog.golang.org/2011/09/go-image-package.html

Update #2547

R=adg, nigeltao
CC=golang-dev
https://golang.org/cl/5933049

doc/progs/image_package6.go

@@ -0,0 +1,17 @@
+// Copyright 2012 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 main
+
+import (
+	"fmt"
+	"image"
+)
+
+func main() {
+	m0 := image.NewRGBA(image.Rect(0, 0, 8, 5))
+	m1 := m0.SubImage(image.Rect(1, 2, 5, 5)).(*image.RGBA)
+	fmt.Println(m0.Bounds().Dx(), m1.Bounds().Dx()) // prints 8, 4
+	fmt.Println(m0.Stride == m1.Stride)             // prints true
+}