Follow up: Added 'Rails/HasAndBelongsToMany' to the StyleGuide.

.rubocop.yml

@@ -146,6 +146,8 @@ Rails/FindEach:
   Enabled: false
 
 Rails/HasAndBelongsToMany:
+  Description: 'Prefer has_many :through to has_and_belongs_to_many.'
+  StyleGuide: 'https://github.com/bbatsov/rails-style-guide#has-many-through'
   Enabled: false