Add link from gitlab-housekeeper readme to blueprint

gems/gitlab-housekeeper/README.md

 # Gitlab::Housekeeper
 
+Check out [the original
+blueprint](https://docs.gitlab.com/ee/architecture/blueprints/gitlab_housekeeper/)
+for the motivation behind the `gitlab-housekeeper`.
+
 This is a gem which can be run locally or in CI to do static and dynamic
 analysis of the GitLab codebase and, using a list of predefined "keeps", it will
 automatically create merge requests for things that developers would have