Generate 'Comment' model and migrate database.

 * generate comment model: rails generate model Comment
 author_name:string body:text article:references
  - This command creates (1) a new migration file:
    db/migrate/xxx_create_comments.rb and creates a new model for our
    comments: app/models/comment.rb
 * migrate database: rake db:migrate
  - This command creates the table in our database and updates the
    db/schema.rb file.