Add rubocop config file back to gem release files

[Burgestrand]

Closes #865

We changed which files are included in the gem recently, this was done to modernize our gemspec along with what bundle gem generated.

bundle gem mygem --exe --coc --changelog --github-username=burgestrand --mit --test=rspec --ci=github --linter=standard

spec.files = Dir.chdir(__dir__) do
    `git ls-files -z`.split("\x0").reject do |f|
      (File.expand_path(f) == __FILE__) ||
        f.start_with?(*%w[bin/ test/ spec/ features/ .git .github appveyor Gemfile])
    end
  end

That change accidentally removed a config file we point rubocop to in our README.

To do

• I have read the contributing guidelines.
• I have added relevant tests.
• I have adjusted relevant documentation.
• I have made sure the individual commits are meaningful.
• I have added relevant lines to the CHANGELOG.

PS: Thank you for contributing to Pundit ❤️