add environmental file to allow force/deny installer app

.env.php

@@ -0,0 +1,4 @@
+<?php
+putenv('EE_INSTALL_MODE=FALSE');
+
+// EOF

README.md

@@ -24,6 +24,11 @@ But ExpressionEngine _can_ run on PHP 5.6+ and MySQL 5.5.3+. For full details an
 
 Read [Installing ExpressionEngine](https://docs.expressionengine.com/latest/installation/installation.html) in the User GUide for full instructions, tips, and post-install best practices.
 
+Note: If you're installing from the repository, you will also need to:
+
+- add an empty config file, e.g. `touch system/user/config.php`
+- route requests to the installer app instead of the main app by changing `EE_INSTALL_MODE` to `TRUE` in [.env.php](.env.php). You can change this back when you're done.
+
 ## How to Get Started
 
 ExpressionEngine separates your content from your design, enabling you to make small or large changes to your website with ease. Your content is stored in Channels, and your design is kept in Templates, both of which are entirely under your control. ExpressionEngine bends to _your_ needs, not the other way around like many other CMSes.

admin.php

@@ -108,6 +108,16 @@
 	// The control panel access constant ensures the CP will be invoked.
 	define('REQ', 'CP');
 
+	// force the installer/updater?
+	if (file_exists(FCPATH.'.env.php') && (require FCPATH.'.env.php') == TRUE && getenv('EE_INSTALL_MODE') === 'TRUE')
+	{
+		define('INSTALL_MODE', TRUE);
+	}
+	else
+	{
+		define('INSTALL_MODE', FALSE);
+	}
+
 /*
  * --------------------------------------------------------------------
  *  Set the error reporting level
@@ -123,7 +133,6 @@
 		error_reporting(0);
 	}
 
-
 /*
  *---------------------------------------------------------------
  * LOAD THE BOOTSTRAP FILE

system/ee/EllisLab/ExpressionEngine/Boot/boot.php

@@ -66,7 +66,11 @@
  */
 	use EllisLab\ExpressionEngine\Core;
 
-	if (FALSE && defined('REQ') && in_array(REQ, ['CP', 'CLI']) && is_dir(SYSPATH.'ee/installer/'))
+	if (
+		defined('REQ') && in_array(REQ, ['CP', 'CLI']) &&
+		is_dir(SYSPATH.'ee/installer/') &&
+		( ! defined('INSTALL_MODE') OR INSTALL_MODE != FALSE)
+	)
 	{
 		$core = new Core\Installer();
 	}