Guide developers through getting sign-in working

[zwaap]

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.