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.
What Stof means is that there is a getParent method defined on the FormTypeInterface which is a better approach to inheriting from other form types.

Basically, instead of extending a BaseType and extending it (And calling parent:: methods), you extend AbstractType and override getParent() which returns the name of the form type you want to extend, in this case `fos_user_registration`.