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 should be removed IMO. The user already saw the main page when going to other doc pages (or at least they should). We don't describe the installation again on each page