Adds Dangerfile for rubygems

What does this MR do and why?

Link to https://docs.gitlab.com/ee/development/gemfile.html when Danger detects a change to Gemfile, or Gemfile.lock.

This will be helpful as https://docs.gitlab.com/ee/development/gemfile.html contains many important things to look out for.

Screenshots or screen recordings

How to set up and validate locally

1. Commit a change to Gemfile.
2. Run bundle exec rake danger_local

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.