feat: new configuration system (and schema) (#2819)

CONTRIBUTING.md

@@ -24,7 +24,7 @@ bundle install
 
 ## Configuration
 
-At present, configuration is handled using a config file. This lives in `config/postal/postal.yml`. An example configuration file is provided in `config/postal.example.yml`. This example is for development use only and not an example for production use.
+Configuration is handled using a config file. This lives in `config/postal/postal.yml`. An example configuration file is provided in `config/examples/development.yml`. This example is for development use only and not an example for production use.
 
 You'll also need a key for signing. You can generate one of these like this:
 
@@ -32,6 +32,10 @@ You'll also need a key for signing. You can generate one of these like this:
 openssl genrsa -out config/postal/signing.key 2048
 ```
 
+If you're running the tests (and you probably should be), you'll find an example file for test configuration in `config/examples/test.yml`. This should be placed in `config/postal/postal.test.yml` with the appropriate values.
+
+If you prefer, you can configure Postal using environment variables. These should be placed in `.env` or `.env.test` as apprpriate.
+
 ## Running
 
 The neatest way to run postal is to ensure that `./bin` is your `$PATH` and then use one of the following commands.