Add instructions about how to support localhost

Summary:
With T764, http://localhost doesn't work anymore. So add instructions
about how to support it by modifying the hosts file.

Test Plan:

• turned on setup mode and the error message did show up
• turned off the setup mode and the error message also showed up

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T764

Differential Revision: https://secure.phabricator.com/D1370