minor symfony#8385 Add a note about using Composer to install Encore (fabpot)

This PR was merged into the 3.3 branch.

Discussion
----------

Add a note about using Composer to install Encore

Commits
-------

6844538 added a note about using Composer to install Encore

Author: xabbuh

frontend/encore/installation.rst

@@ -14,6 +14,14 @@ Then, install Encore into your project with Yarn:
     If you want to use `npm`_ instead of `yarn`_, replace ``yarn add xxx --dev`` by
     ``npm install xxx --save-dev``.
 
+.. tip::
+
+    If you are using Flex for your project, you can install Encore via:
+
+    .. code-block:: terminal
+
+        $ composer require encore
+
 This command creates (or modifies) a ``package.json`` file and downloads dependencies
 into a ``node_modules/`` directory. When using Yarn, a file called ``yarn.lock``
 is also created/updated. When using npm 5, a ``package-lock.json`` file is created/updated.