Expand documentation for users

[csj1012]

Improve/add documentation re: how to use Mannequin once it's set up:

Drupal:

• Examples of Drupal twig markup, yml usage (in same file), etc.
• Serving Mannequin in the context of a hosting environment like Pantheon or Acquia
• Flexibility of abstraction layer

General:

• CI integration

Suggestions welcome!