adding note that CSRF protection has to be enabled in config

JensPliester · xabbuh · commit b948747a2211 · 2017-07-11T19:26:26.000+02:00
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.
diff --git a/security/csrf_in_login_form.rst b/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
 ------------------------
