Improve --domains documentation

Closes #432

README.md

@@ -93,7 +93,7 @@ COMMANDS:
      help, h  Shows a list of commands or help for one command
 
 GLOBAL OPTIONS:
-   --domains value, -d value   Add domains to the process
+   --domains value, -d value   Add a domain to the process. Can be specified multiple times.
    --csr value, -c value       Certificate signing request filename, if an external CSR is to be used
    --server value, -s value    CA hostname (and optionally :port). The server certificate must be trusted in order to avoid further modifications to the client. (default: "https://acme-v01.api.letsencrypt.org/directory")
    --email value, -m value     Email used for registration and recovery contact.

cli.go

@@ -109,7 +109,7 @@ func main() {
 	app.Flags = []cli.Flag{
 		cli.StringSliceFlag{
 			Name:  "domains, d",
-			Usage: "Add domains to the process",
+			Usage: "Add a domain to the process. Can be specified multiple times.",
 		},
 		cli.StringFlag{
 			Name:  "csr, c",