acme/autocert: add Cache to Manager example

If you don't use a cache you're just begging for Let's Encrypt
to ban you for a week due to duplicate certificate requests.
Don't encourage users to get banned.

Change-Id: Icf999c7f28817990bb45931d30692d4040c0ed71
Reviewed-on: https://go-review.googlesource.com/66550
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Vaghin <ddos@google.com>

acme/autocert/autocert.go

@@ -83,8 +83,10 @@ func defaultHostPolicy(context.Context, string) error {
 // It obtains and refreshes certificates automatically,
 // as well as providing them to a TLS server via tls.Config.
 //
-// To preserve issued certificates and improve overall performance,
-// use a cache implementation of Cache. For instance, DirCache.
+// You must specify a cache implementation, such as DirCache,
+// to reuse obtained certificates across program restarts.
+// Otherwise your server is very likely to exceed the certificate
+// issuer's request rate limits.
 type Manager struct {
 	// Prompt specifies a callback function to conditionally accept a CA's Terms of Service (TOS).
 	// The registration may require the caller to agree to the CA's TOS.

acme/autocert/example_test.go

@@ -23,6 +23,7 @@ func ExampleNewListener() {
 
 func ExampleManager() {
 	m := autocert.Manager{
+		Cache:      autocert.DirCache("secret-dir"),
 		Prompt:     autocert.AcceptTOS,
 		HostPolicy: autocert.HostWhitelist("example.org"),
 	}