Add command to require Symfony security

The command should be there, because if someone unfamiliar with new Symfony structure use it, it will not work.

Author: Holicz

security/entity_provider.rst

@@ -35,7 +35,12 @@ and :ref:`user serialization to the session <security-serialize-equatable>`
 
 1) Create your User Entity
 --------------------------
+Firstable you need to require a Symfony security
 
+.. code-block:: terminal
+
+    $ composer req security
+    
 For this entry, suppose that you already have a ``User`` entity inside an
 ``AppBundle`` with the following fields: ``id``, ``username``, ``password``,
 ``email`` and ``isActive``::