feature symfony#5355 Added a mention to the Symfony Demo application (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
----------

Added a mention to the Symfony Demo application

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -

Commits
-------

5482fda Fixed some issues reported by Wouter
d9f20c2 Fixed minor issue with the title case
b3a644a Added a mention to the Symfony Demo application

Author: weaverryan

book/installation.rst

@@ -110,7 +110,7 @@ pass ``lts`` as the second argument of the ``new`` command:
     $ symfony new my_project_name lts
 
     # Windows
-    c:\projects\> php symfony.phar new my_project_name lts
+    c:\projects\> php symfony new my_project_name lts
 
 Read the :doc:`Symfony Release process </contributing/community/releases>`
 to better understand why there are several Symfony versions and which one
@@ -302,6 +302,30 @@ them all at once:
 Depending on the complexity of your project, this update process can take up to
 several minutes to complete.
 
+Installing the Symfony Demo Application
+---------------------------------------
+
+The Symfony Demo application is a fully-functional application that shows the
+recommended way to develop Symfony applications. The application has been
+conceived as a learning tool for Symfony newcomers and its source code contains
+tons of comments and helpful notes.
+
+In order to download the Symfony Demo application, execute the ``demo`` command
+of the Symfony Installer anywhere in your system:
+
+.. code-block:: bash
+
+    # Linux, Mac OS X
+    $ symfony demo
+
+    # Windows
+    c:\projects\> php symfony demo
+
+Once downloaded, enter into the ``symfony_demo/`` directory and run the PHP's
+built-in web server executing the ``php app/console server:run`` command. Access
+to the ``http://localhost:8000`` URL in your browser to start using the Symfony
+Demo application.
+
 .. _installing-a-symfony2-distribution:
 
 Installing a Symfony Distribution