Add :key option to session config example.

I have experienced strange behavior with the flash in Rails 4.1.1 when not defining the `:key` on the `config.session_store`. The strange behavior includes the flash not being set, is lost, or not appearing in the correct context (not on the following action but one sometimes two actions later).

I've yet to pinpoint _why_ this is happening and will open an issue/pr when I do.
By default Rails generates a key named `_[app_name]_session` in
`session_store.rb`. Since the app is name `Foo` in the example I stuck with
the Rails convention.

Happy to re-submit this pull request with a suggestion for a better key
name.

Thanks.


On Mon, Jun 16, 2014 at 3:13 PM, Arthur Nogueira Neves <
notifications@github.com> wrote:

> In README.md:
>
> > @@ -21,7 +21,7 @@ Run the migration generator:
> >  Then, set your session store in `config/initializers/session_store.rb`:
> >
> >  ```ruby
> > -Foo::Application.config.session_store :active_record_store
> > +Foo::Application.config.session_store :active_record_store, :key => '_foo_session'
>
> I would add a better :key namespace label in here. Given that this is the
> docs.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/rails/activerecord-session_store/pull/31/files#r13824707>
> .
>