Update documentation of begin_of_association_chain

Imagine you have the following:

```Ruby
def begin_of_association_chain
  current_user.billing_record
end
```

Because of how begin_of_association_chain is implemented, if for some reason ```current_user.billing_record``` happens to be ```nil``` then your user will end up having access to every object because that's what the default implementation returns.

I propose adding something about this in the documentation because it's something that one might not realize until a user tells him.

I think it probably should also be noted in the README since it's not an obvious thing at first IMHO and many people might have not realized it, but I'd like to get your opinion before going any further... (in fact, the text is almost a draft)

cheers.