Enhance danger bot documentation

- this note would have been helpful when setting up danger
  on versions app

doc/development/dangerbot.md

@@ -95,6 +95,12 @@ through in CI. However, you can speed these cycles up somewhat by emptying the
 `.gitlab/ci/rails.gitlab-ci.yml` file in your merge request. Just don't forget
 to revert the change before merging!
 
+To enable the Dangerfile on another existing GitLab project, run the following extra steps, based on [this procedure](https://danger.systems/guides/getting_started.html#creating-a-bot-account-for-danger-to-use):
+
+1. Add `@gitlab-bot` to the project as a `reporter`.
+1. Add the `@gitlab-bot`'s `GITLAB_API_PRIVATE_TOKEN` value as a value for a new CI/CD
+   variable named `DANGER_GITLAB_API_TOKEN`.
+
 You should add the `~Danger bot` label to the merge request before sending it
 for review.