acme/autocert: extract Manager example from comments

Ensures that it compiles and makes it more discoverable in godoc.

Change-Id: I50a3deb23110017d0eb16b1da0ac729a96ff2602
Reviewed-on: https://go-review.googlesource.com/40952
Run-TryBot: Alex Vaghin <ddos@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/acme/autocert/example_test.go b/acme/autocert/example_test.go
new file mode 100644
index 0000000..c6267b8
--- /dev/null
+++ b/acme/autocert/example_test.go
@@ -0,0 +1,34 @@
+// Copyright 2017 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 autocert_test
+
+import (
+	"crypto/tls"
+	"fmt"
+	"log"
+	"net/http"
+
+	"golang.org/x/crypto/acme/autocert"
+)
+
+func ExampleNewListener() {
+	mux := http.NewServeMux()
+	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprintf(w, "Hello, TLS user! Your config: %+v", r.TLS)
+	})
+	log.Fatal(http.Serve(autocert.NewListener("example.com"), mux))
+}
+
+func ExampleManager() {
+	m := autocert.Manager{
+		Prompt:     autocert.AcceptTOS,
+		HostPolicy: autocert.HostWhitelist("example.org"),
+	}
+	s := &http.Server{
+		Addr:      ":https",
+		TLSConfig: &tls.Config{GetCertificate: m.GetCertificate},
+	}
+	s.ListenAndServeTLS("", "")
+}