adding note that CSRF protection has to be enabled in config

This issue should be mentioned in the guide.
If CSRF Protection is disabled, Symfony won't find the CSRF-Token manager, implicating a missing package.
But all was needed was setting csrf_protection to true in config.yml.

Author: JensPliester

security/csrf_in_login_form.rst

@@ -74,6 +74,7 @@ provider available in the Security component:
 
 The Security component can be configured further, but this is all information
 it needs to be able to use CSRF in the login form.
+Note: Make sure that CSRF Protection is enabled in ``config.yml``.
 
 Rendering the CSRF field
 ------------------------