Add Style Guide to CONTRIBUTING.md

[ChaelCodes]

Describe the Feature

Is your feature request related to a problem? Please describe.
There are a few things that I consider Antipatterns in the Rails community, like tests on model validations. These aren't catchable by Rubocop, and if they're part of a PR, I'd let them slide, but I do want to guide others away from using them.

Describe the Technical Implementation of the Solution

Solution
Add a section to the CONTRIBUTING.md called a style guide and mention uncommon decisions there.

Please comment on this issue with thought and recommendations for the Style Guide.