add support for rubocop

[Kallin]

This issue is a (choose one):

• [X ] Feature request.

Checklist before submitting:

• I've searched for an existing issue.
• I've asked my question on Gitter and have not received a satisfactory answer.
• I've included a complete bug report template. This step helps us and allows us to see the bug without trying to reproduce the problem from your description. It helps you because you will frequently detect if it's a problem specific to your project.
• The feature I'm asking for is compliant with the JSON:API spec.

Description

I'd like to add support for rubocop to the project. We could also consider adding checks to PRs in the future to enforce rubocop checks once we've gotten the master branch clean.