IACO-551 document preference view

Added a section about using the preferences view.
This will probably be easier to view as rendered by github [here](https://github.com/yuval-netanel/inapp-ios/blob/feature/IACO-551-document-preference-view/doc/integration.md#the-preferences-view).
I'd replace "[self getUserToken]" with "userToken", and tell the developer to assume that userToken is the previously-stored user token.