Add changelog generation to ruby sdk

Problem

The changelog for new releases should already be generated automatically thanks to the inclusion of default GitLab gems CI setup, but it's neither documented nor verified working right now.

Desired Outcome

For the changelog generation on new releases:

• test it to make sure it works
• document in the README how to use it

Proposed Solution

Look at the docs for https://docs.gitlab.com/ee/development/gems.html#create-and-publish-a-ruby-gem