minor symfony#102 Updated the Global Composer Installation article (javiereguiluz)

This PR was merged into the 2.7 branch.

Discussion
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Updated the Global Composer Installation article

Recently we discussed about this. I was one of the people in favor of keeping our article because the installation proposed by Composer is more complex. However, I was wrong. The official Composer doc is the way to go to install Composer. It doesn't matter if it's a bit more complex because this is something you do once in your lifetime.

Commits
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cacfb88 Updated the Global Composer Installation article

Author: javiereguiluz

setup/composer.rst

@@ -5,33 +5,11 @@ Installing Composer
 ===================
 
 `Composer`_ is the package manager used by modern PHP applications. Use Composer
-to manage dependencies in your Symfony applications and to install Symfony Components
-in your PHP projects.
+to manage dependencies in your Symfony applications and to install Symfony
+Components in your PHP projects.
 
-It's recommended to install Composer globally in your system as explained in the
-following sections.
+Since this article was first published, Composer installation has improved a lot.
+Therefore, the original contents of this article have been removed and you are
+encourage to `install Composer`_ as explained in the official Composer documentation.
 
-Install Composer on Linux and Mac OS X
---------------------------------------
-
-#. Run the installer as described in `the official Composer documentation`_;
-#. Execute the following command to move the ``composer.phar`` to a directory that is in your path:
-
-   .. code-block:: bash
-
-       $ sudo mv composer.phar /usr/local/bin/composer
-
-Install Composer on Windows
----------------------------
-
-Download the installer from `getcomposer.org`_, execute it and follow the instructions.
-
-Learn more
-----------
-
-Read the `Composer documentation`_ to learn more about its usage and features.
-
-.. _`Composer`: https://getcomposer.org/
-.. _`Composer documentation`: https://getcomposer.org/doc/00-intro.md
-.. _`getcomposer.org`: https://getcomposer.org/Composer-Setup.exe
-.. _the official Composer documentation: https://getcomposer.org/download
+.. _`install Composer`: https://getcomposer.org/download/