minor #309 Display a warning message when installing the Demo application (javiereguiluz)

This PR was merged into the 1.0-dev branch.

Discussion
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Display a warning message when installing the Demo application

This fixes #308 and it looks like this:

![warning-demo-project](https://user-images.githubusercontent.com/73419/38092665-3595370e-3369-11e8-91a8-0a9e24955770.png)

Commits
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9de3bbe Display a warning message when installing the Demo application

Author: javiereguiluz

src/Symfony/Installer/DemoCommand.php

@@ -157,6 +157,13 @@ private function displayInstallationResult()
             $this->projectDir, $serverRunCommand
         ));
 
+        $this->output->writeln(
+            "<bg=yellow> WARNING </>\n\n".
+            "   This installer downloads the old Symfony Demo version based on Symfony 3.\n".
+            "   If you prefer to install the new version based on Symfony 4 and Symfony Flex,\n".
+            "   execute the following command:\n\n".
+            "     composer create-project symfony/symfony-demo\n");
+
         return $this;
     }