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.
I'll process the other comments, but I'm not sure what you mean here.
I followed the official doc => [Overriding Forms](https://github.com/FriendsOfSymfony/FOSUserBundle/blob/master/Resources/doc/overriding_forms.md)