Add best practices for Ruby gems

Content is from old thoughtbot development handbook.
Avoid bypassing Rails validations

Extracted from old thoughtbot development handbook.
Clean up Bundler guidelines

* Remove old Bundler recommendation in Rails section, which has been
  superceded by the Bundler section.
* Alphabetize Bundler guidelines.
* Wrap at 80 characters where possible.
* Add binstubs guideline.