note production url

README.md

@@ -103,6 +103,7 @@ now here's the switch:
 
 // returns an instance of node-letsencrypt with additional helper methods
 var lex = require('letsencrypt-express').create({
+  // set to https://acme-v01.api.letsencrypt.org/directory in production
   server: 'staging'
 
 // If you wish to replace the default plugins, you may do so here