Guide developers through getting sign-in working

In order for sign-in to work, `Configuration.AppClientID` must be specified with a valid developers.arcgis.com client ID, and the value specified for `Configuration.RedirectURL` must be specified in the list of redirect URIs for the application. In trying to get this working locally, this didn't seem clear. I would suggest:

1. Spelling this out in the doc in an easily discoverable way.
2. Adding a check during sign-in to point the developer to these values. This could, for instance, check for the default value of `AppClientID` and call that out specifically if unchanged, and otherwise suggest checking these values on sign-in failure.