Add docs for version.gitlab.com creation

doc/rake-tasks.md

@@ -99,6 +99,8 @@ This task will:
 1. Create the `v[version]` and `v[version]-ee` tags, pointing to the respective
    branches created above.
 1. Push all newly-created branches and tags to all remotes.
+1. Add an entry to [version.gitlab.com](https://version.gitlab.com) unless the
+   version is pre-release, `TEST` is true, or either package is skipped.
 ### Configuration