build.sh requires drupalorg_drush but doesn't say so

Problem/Motivation

When I try to run build.sh on my local machine, it errors out because it's missing the verify-makefile command:

mparker17@mcomp7:(drupal)/panopoly (7.x-1.x ✓) % ./build.sh /Users/mparker17/panopoly
 ___   ___   ___
|___| |   | |   |
 ___  |   | |___|
|   | |   |  ___ 
|   | |___| |___|
|   |  _________ 
|___| |_________|

=================
    Panopoly     
=================

This command can be used to build the distribution.

  [1] Build distribution at /Users/mparker17/panopoly (in release mode)
  [2] Build distribution at /Users/mparker17/panopoly (in development mode)

Selection: 1
Building Panopoly distribution...
Verifying make...
The drush command 'verify-makefile' could not be found.  Run `drush cache-clear drush` to clear the commandfile cache if you have installed new extensions. [error]
A Drupal installation directory could not be found [error]

After a brief chat in IRC with @mglaman and @dsnopek, we found this is because I hadn't installed the Drupal.org drush → project on my local machine.

A quick grep shows the only mention of it is in the travis build script:

mparker17@mcomp7:(drupal)/panopoly (7.x-1.x ✓) % rgrep 'drupalorg_drush' .
./scripts/travis-ci.sh:32:  drush dl -y drupalorg_drush-7.x-1.x-dev --destination=$HOME/.drush

I feel like this should be documented somewhere.

Proposed resolution

There doesn't appear to be any existing prose documentation on the build.sh script...

mparker17@mcomp7:(drupal)/panopoly (7.x-1.x ✓) % rgrep -w 'build.sh' .
mparker17@mcomp7:(drupal)/panopoly (7.x-1.x ✓) %

... so I propose adding documentation and/or a check to the build.sh script.

Remaining tasks

1. Write a patch
2. Review and RTBC
3. Commit

User interface changes

When running build.sh, it prints a message to tell you to install the Drupal.org drush → project (ideally, if the verify-makefile command does not exist, ideally before erroring out).

API changes

None.