Update docs to warn about subscribing directly to state$

Feature request for docs

What is the current behavior? Subscribing to the state$ can be problematic if you later rely on the current value of the action$. It is intended that the state$ emits first, as most examples suggest starting with a subscription to the action$, however if someone chooses to subscribe to the state$ they should be away that pulling in the current action$ value with combineLatest() will be incorrect (it will be the previous value).

state$.pipe(
    map(state => state.accountBalance),
    distinctUntilChanged(),
    withLatestFrom(action$),
    tap(([balance, action]) => {
        console.log('The reduces updated the balance in response to ', action.type); // This is actually the previous value...
    })
)

What is the expected behavior? This is expected (I can't think of a logical alternative), however it's not obvious and can be really confusing.

Suggested work around in the docs:

state$.pipe(
    sample(action$),
    map(state => state.accountBalance),
    distinctUntilChanged(),
    withLatestFrom(action$),
    tap(([balance, action]) => {
        console.log('The reduces updated the balance in response to ', action.type); // The action is correct
    })
)

The above is slightly nicer than:

action$.pipe(
    withLatestFrom(state$),
    pick('1')
);

or you could (but a less reactive approach)

action$.pipe(
    mapTo(state$.value)
);

Which versions of redux-observable, and which browser and OS are affected by this issue? Did this work in previous versions of redux-observable? All