Add guides to rbis

[adisonlampert]

Motivation

Worked on this during group pairing last week and today. https://github.com/Shopify/ruby-dev-exp-issues/issues/642

Implementation

Added comments to create_method so that guides could be passed into RBI::Method.new which already takes comments as a parameter.

This a screenshot from code-db in app/models/project.rb Here, "Rails Guides" links to the official Rails guide on has_many associations since a project has_many snapshots.

Tests

Test haven't been updated yet.

Thoughts?