Add documentation on starting the machine

Took me a while to realize that calling useMachine does not start the machine. Is there a good reason to leave that up to the caller? The machine is stopped when unmounting, so maybe it makes sense to start it when it is first called? If not, then it would be nice to document this. FWIW, I had to add this code to start my machine:

const machine = useMachine(...);

useEffect(() => {
  machine.service.start();
}, []);

[bschlenk]

I found the issue. onTransition and onChange are called after init, so anything that happens during initialization is missed. Calling init after both callbacks are set fixes the problem.

[bschlenk]

I created a minimal example, but this may actually be an issue with xstate, it looks like it isn’t calling onChange for actions triggered during initialization.

https://codesandbox.io/s/4zjv035mo9

I took your example and wrapped it in two outer states: setup and main. I also added a name field to the context that defaults to "unset". Setup has an onEntry action that updates name, and then an immediate transition to main. If you look at the output, name doesn’t get updated as expected. But if you uncomment the machine.service.start call on line 86, the state does update.

If you leave the start call commented and click one of the tick buttons, you can see that name does update. So the state machine is holding the correct state, it just isn’t notifying react that the state context has changed during initialization.