added a note about using Composer to install Encore

Author: fabpot

frontend/encore/installation.rst

@@ -25,6 +25,14 @@ is also created/updated. When using npm 5, a ``package-lock.json`` file is creat
 
 Next, create your ``webpack.config.js`` in :doc:`/frontend/encore/simple-example`!
 
+.. tip::
+
+    If you are using Flex for your project, you can install Encore via:
+
+    .. code-block:: terminal
+
+        $ composer require encore
+
 .. _`install Node.js`: https://nodejs.org/en/download/
 .. _`Yarn package manager`: https://yarnpkg.com/lang/en/docs/install/
 .. _`npm`: https://www.npmjs.com/