Develop installation and usage guide for this module

It was noted in ticket #1190290: Lack of documentation that the community is interested in additional documentation regarding the setup and configuration of this module. This ticket is intended to track the development of that documentation.

Please note that this documentation will always be a work-in-progress. Also note that we will focus documentation on the Drupal 7.x release, as it is most in keeping with the features available via MailChimp's API.