Dispatch returns undefined in certain cases.

I brought up this issue in rackt/redux#481.

Im not sure if this can be solved from a code point of view but it might be worth documenting the issue with dispatch result being undefined in certain cases.
- middleware needs to come after thunk, promise or any middleware where you expect a value to be returned.
- with non-thunk actions you will not be able to access the return value.

Perhaps worth listing [redux-batched-subscribe](https://github.com/tappleby/redux-batched-subscribe) as an alternative.