Document the behaviour when deactivating the plugin

[johnbillion]

Need to document what happens when the plugin is deactivated.

• Who is attributed to each post?
  • A: The original author of the post
• Who is able to edit each post?
  • A: The original author of the post, plus Administrators and Editors
• What happens to Guest Authors?
  • They can now log in
  • They cannot perform any action, they have no capabilities
  • The role will be retained in the database unless it's explicitly removed with remove_role()