Improved documentation for supported Redux middleware

[mpeyper]

It was suggested in #8 that we could improve the documentation around support, limitations and incompatibility of Redux middleware.

As it stands, redux-thunk is the only supported middleware and there is a known limitation with redux-sagas (#18).

Any suggestions on the format of documentation (or a pull request) are welcome.

[mpeyper]

Closing this as there is now a section in the docs for middleware and improved middleware support introduced in #34.