Improve install documentation a bit

Summary:
  - Warn users that they'll need to be comfortable with the CLI.
  - Move XHProf stuff to the developer docs, since few/no normal users need it.

Test Plan: Read documentation.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10810