README incorrect with regards to current_user

> If the method to derive the current user is called something other than current_user, you will also want to configure current_user_method.

https://github.com/hyperoslo/feeder/blob/master/lib/feeder/configuration.rb#L14:

```
def initialize
@test_mode = false
@observables = {}
@authorization_adapter = Feeder::AuthorizationAdapters::Base
@current_user_method = nil
@like_scopes = []
end
```

I'd be in favour of reverting #26 and at runtime determine whether the `current_user_method` is available, as opposed to forcing people to configure it.

If we keep it as it is, we need to update the README at the very least.