Clarify why a ViewModel should delegate data access to a service

Follow-up to [this issue](https://github.com/johnpapa/angularjs-styleguide/issues/181#issuecomment-67099471) where the issuer wanted code that would work.

What this pull request does instead is show, with comments and pseudo-code, how many steps might be necessary if the ViewModel were to attempt  the job itself instead of delegating to a service.

None of these steps are things the ViewModel should know how to do.