Document configuration overload order

[pictographer]

We use the config-rs crate and the clap crate to allow settings to be overridden. Settings come from the following sources in "last one wins" order:

• Default values in the Rust code
• Tide Disco's configuration file
• The organization's configuration file
• The application's configuration file
• The command line arguments
• Environment variables

This is documented in tide-disco/src/lib.rs::compose_settings, but the documentation is a bit out-of-date. We now have the espresso-systems-common crate which should have the organization's config file.