Add note how to get previous on impersonation

[Koc]

http://symfony.com/doc/master/cookbook/security/impersonating_user.html
http://stackoverflow.com/questions/6371712/how-to-identify-if-a-user-is-being-impersonated-in-symfony2

[weaverryan]

Nice idea - a quick note that switched users have a special ROLE_PREVIOUS_ADMIN role would be great.

[Sander-Toonen]

There already is a note about the existence of the ROLE_PREVIOUS_ADMIN role. I think @Koc means adding a note about getting the user object of the impersonator.

[Koc]

Right