Passing state from signinRedirect to successCallback

It would be helpful to document somewhere that it is possible to pass serialized data from signinRedirect through the login redirects to CallbackComponent.successCallback.

```
userManager.signinRedirect({ data: 'testing 123' })
```
results in
```
<CallbackComponent
  successCallback={user => {
    console.log(user.state)        // outputs 'testing 123'
  }}
  ...
/>
```