Update issue_weight.md with EE tier

doc/workflow/issue_weight.md

 # Issue Weight
 
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/76)
+in [GitLab Enterprise Edition Starter](https://about.gitlab.com/gitlab-ee/) 8.3.
+
 When you have a lot of issues, it can be hard to get an overview.
 By adding a weight to each issue, you can get a better idea of how much time,
 value or complexity a given issue has or will cost.