README: Add hint about NextProtos for certmagic.TLS

[oliverpool]

This hint is already present a couple of lines later, however I overlooked it in the large advanced usescase.

In my usecase I just want to customize the storage:

		// SETUP TLS
		certmagic.DefaultACME.Email = "redacted@example.org"
		certmagic.DefaultACME.Agreed = true
		certmagic.DefaultACME.DisableHTTPChallenge = true

		// bugfix will come here

		certConfig := certmagic.NewDefault()
		certConfig.Storage = &certmagic.FileStorage{
			Path: stateDir + ".certmagic",
		}
		tlsConfig := certConfig.TLSConfig()
		tlsConfig.NextProtos = append([]string{"h2", "http/1.1"}, tlsConfig.NextProtos...)

		ln, err = tls.Listen("tcp", addr, tlsConfig)
		if err != nil {
			return err
		}

		if err := certConfig.ManageAsync(ctx, domains); err != nil { // async to prevent systemd restart
			return fmt.Errorf("could not manage TLS certificates: %v", err)
		}

[mholt]

makes sense -- thanks for the enhancement!