update the docs for per request config

[jonchurch]

We get issues occassionally asking about how to do per route configuration of various sorts.

We do support this, but the readme doesn't do a good job of signaling.

Updated the readme to make it clearer, and tweaked the example to show a more appropriate example. The previous example can be accomplished by passing a function as the origin option, the updated example shows setting credentials because that is the only way to set it per request currently.