document that Tome export base URL needs to be in trusted_host_patterns

Problem/Motivation

I used the export form at admin/config/tome/static/generate and immediately got a white error page saying that the domain I had just entered wasn't an authorized domain.

This needs to be added to $settings['trusted_host_patterns'] in settings.php -- or could Tome step in and add this on the fly?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes