Elaborate in readme where OpsGenie handler config should go

[Neil Hooey]

The "Usage" section just pastes a fragment of JSON without saying where that JSON should go. The [https://sensuapp.org/docs/latest/enterprise-built-in-handlers#opsgenie](enterprise OpsGenie handler documentation) does the same thing.

If you put the configuration under the handlers: key in /etc/sensu/conf.d/handlers.json, Sensu 0.21.0-2 starts up and says: unknown handler type.

Please provide more details about how to configure this handler.