Merge branch 'patch-9' into 'master'

Update CONTRIBUTING.md - Adding info about issues within a milestone

## What does this MR do?
Adds copy regarding issues within the current milestone to the contribution guide.

## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

cc @dzaporozhets @awhildy

See merge request !7516

CONTRIBUTING.md

@@ -216,7 +216,10 @@ associated with in the description of the issue.
 We welcome merge requests with fixes and improvements to GitLab code, tests,
 and/or documentation. The features we would really like a merge request for are
 listed with the label [`Accepting Merge Requests` on our issue tracker for CE][accepting-mrs-ce]
-and [EE][accepting-mrs-ee] but other improvements are also welcome.
+and [EE][accepting-mrs-ee] but other improvements are also welcome. Please note
+that if an issue is marked for the current milestone either before or while you 
+are working on it, a team member may take over the merge request in order to 
+ensure the work is finished before the release date.
 
 If you want to add a new feature that is not labeled it is best to first create
 a feedback issue (if there isn't one already) and leave a comment asking for it