style: mention to call generate:manifest command in command docblock

Author: maxgalbu

templates/command.txt

@@ -14,13 +14,15 @@ export default class {{ filename }} extends BaseCommand {
   public static settings = {
     /**
      * Set the following value to true, if you want to load the application
-     * before running the command
+     * before running the command. Don't forget to call `node ace generate:manifest` 
+     * afterwards.
      */
     loadApp: false,
 
     /**
      * Set the following value to true, if you want this command to keep running until
-     * you manually decide to exit the process
+     * you manually decide to exit the process. Don't forget to call 
+     * `node ace generate:manifest` afterwards.
      */
     stayAlive: false,
   }