Documentation to create "email only" registration

As discussed in https://github.com/FriendsOfSymfony/FOSUserBundle/issues/1516 creating a authentication scheme based only on Email/Password is very common.
This is a quick extension to FOSUser documentation to setup such authentication.

I hope this is useful for others.
This is not related to this chapter at all IMO (documenting it is a good idea, but it should go in the main documentation IMO. Please submit it as a separate PR)