add user cli help text

lib/api/user.js

@@ -35,6 +35,13 @@ module.exports = {
             "${command} user invite <address>..."
         ];
     },
+    getHelp: function(){
+        return [
+            "User:",
+            "  Commands:",
+            "    invite <address>...  invite the given list of e-mail addresses to your datadog org"
+        ];
+    },
     handleCli: function(subcommand, args, callback){
         invite(args._.slice(4), callback)
     }