encoding/pem: add Decode example

For #16360.

Change-Id: I99d1e5ab1f814f65b3066a498158a442f1bd477f
Reviewed-on: https://go-review.googlesource.com/31137
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/pem/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 pem_test
+
+import (
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+	"log"
+)
+
+func ExampleDecode() {
+	var pubPEMData = []byte(`
+-----BEGIN PUBLIC KEY-----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+-----END PUBLIC KEY-----
+and some more`)
+
+	block, rest := pem.Decode(pubPEMData)
+	if block == nil || block.Type != "PUBLIC KEY" {
+		log.Fatal("failed to decode PEM block containing public key")
+	}
+
+	pub, err := x509.ParsePKIXPublicKey(block.Bytes)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("Got a %T, with remaining data: %q", pub, rest)
+	// Output: Got a *rsa.PublicKey, with remaining data: "and some more"
+}