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_package3.go

@@ -0,0 +1,15 @@
+// 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() {
+	r := image.Rect(2, 1, 5, 5).Add(image.Pt(-4, -2))
+	fmt.Println(r.Dx(), r.Dy(), image.Pt(0, 0).In(r)) // prints 3 4 true
+}