The current google_compute_url_map resource already supports backend
buckets out of the box: Just pass the self_link of the backend buckets
as you would pass the self_link of a backend service.
This adds some example code as well.
When trying to run the example code with the most recent terraform
master you get the following errors:
```
2 error(s) occurred:
* google_compute_backend_service.home: "region": [REMOVED] region has been removed as it was never used
* google_compute_backend_service.login: "region": [REMOVED] region has been removed as it was never used
```
Hence remove it from the documentation.
