add note to use the constraint UserPassword

Author: zairigimad

reference/constraints/UserPassword.rst

@@ -10,6 +10,11 @@ password, but needs to enter their old password for security.
     This should **not** be used to validate a login form, since this is
     done automatically by the security system.
 
+.. note::
+
+    In order to use this constraints, you should have installed the 
+    symfony/security component with Composer.
+
 ==========  ===================================================================
 Applies to  :ref:`property or method <validation-property-target>`
 Options     - `groups`_