add default web UI login info to README

[jrheling]

Apparently it's admin:admin, which was guessable, but only after some frustrating trial and error.

[jjethwa]

Hi @jrheling

Sorry about that. I follow the Rundeck defaults as much as possible, so I assumed the username and password would be known. I'll update the README on Monday 😃

[jrheling]

Thanks! Obviously it's a small thing, and likely discoverable by anybody who'd be using this.