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.
hmm, which means I should update this doc to use the cleaner way...

I wanted to link to the symfony doc about the form type hierarchy, but looks like it is not yet written :(