Document how to resolve internal routes

[mogul]

This feature was not documented previously. It should be, since putting the app that an nginx proxy points to on an internal route is good practice! So here are some docs.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. Unfortunately, the Pivotal Tracker project is private so you may be unable to view the contents of the story.

The labels on this github issue will be updated when the story is started.

[mjgutermuth]

Thanks @mogul!