Warn users about using bin/accountadmin for first time setup

Summary: The "easy setup" is sort of an exaggeration since it basically just amounts to getting you logged in correctly, but it will probably be more true in the future.

Test Plan: Ran bin/accountadmin with zero and more than zero accounts.

Reviewers: asherkin, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7678