image: add an example that shows how to get the metadata of an image

This is a simple but everyday use case in image libraries. Currently,
there is one example in this library and it is lengthy and involved.

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comment (this text) used to generate the subject and body of
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Change-Id: Idca527d97c095af88755446e1548fa2b8ace7eb0
GitHub-Last-Rev: f5743c8ef337a8b7a5126db21d784593f4f53c8a
GitHub-Pull-Request: golang/go#25616
Reviewed-on: https://go-review.googlesource.com/114939
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/image/decode_example_test.go

@@ -21,6 +21,15 @@ import (
 	_ "image/jpeg"
 )
 
+func Example_decodeConfig() {
+	reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(data))
+	config, format, err := image.DecodeConfig(reader)
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Println("Width:", config.Width, "Height:", config.Height, "Format:", format)
+}
+
 func Example() {
 	// Decode the JPEG data. If reading from file, create a reader with
 	//