Extend readme with environment based configuration
Extend readme with environment based configuration

Environment based configuration is possible in `Hutch` but this was only
apparent if someone took a look into `lib/hutch/config.rb`.

This changeset updates the readme to mention the possibility of this and
also provides a cheat sheet for the configuration preference order.

Fatsoma/hutch,gocardless/hutch,Fatsoma/hutch,Fatsoma/hutch,gocardless/hutch,gocardless/hutch