digitalocean: add ssh instructions

Addresses part of https://twitter.com/shimetweets/status/526336370277892096

> @digitalocean fix the credentials you send in the email for coreos instances. default user is not root, but 'core'. spent hours on this

We can't change their emails, but hopefully this lets people know they can safely ignore them. Also adds a note to optionally set up an ssh-agent for a better fleet experience.
CoreOS is set up to be a little more secure than other DigitalOcean images. By default, it uses the `core` user instead of `root` and doesn't use a password for authentication.