add sensu_agent example with non-default config

[cwjohnston]

Pull Request Checklist

General

• [ ] Update Changelog following the conventions laid out here

• [x] Update README with any necessary configuration snippets

• [ ] Cookstyle (rubocop) passes

• [ ] Foodcritic passes

• [ ] Rspec (unit tests) passes

• [ ] Inspec (integration tests) passes

New Features

• [ ] Added a Testing Artifact as either an automated test or a manual artifact on the PR.

• [ ] Added documentation for it to the README.md

Purpose

Adds an example to the sensu_agent section of the readme which illustrates use of the config parameter to set non-default values for backend-url, subscriptions, labels, annotations, etc.

Known Compatibility Issues

[webframp]

Looks good to me