Add a section on mixin grouping
Add a section on mixin grouping

This change adds a suggested rule on mixin grouping. Some rationales for
this could be:

 - Easier to see the number of dependencies for a `class` or `module`.
 - Easier and less error prone to change or remove a mixin.

Mostly, however, it is for consistency.

voxable-labs/ruby-style-guide